If the grey phantom was the one that crashed then it's officially the unluckiest Gundam ship
Wasn't the Alex supposed to be on white base? So this means the white base crew would be transfered to the grey phantom?
Maybe the Federation "accidentally" crashed the troy horse and coincidentally Zeon soldiers were in the same area it crashed. And the pockets of some federation officers got extra payment

It was the Grey Phantom. According to Mark Simmons the shooting script for Unicorn plainly states that this is the Grey Phantom. Being at Konpei doesn't mean she was sunk there in fact, some ships that were at the Naval Review were also defending the Solar System II at the end of the OVA as stated in the OVA.

Considering too that by Unicorn, 0096/97, the EFSF was busy maintaining their Gryps War-era Salamis Kais, and the Irish and Nahel Argama classes co-opted from the AEUG, along with the much newer Clop and Ra Cailum classes, the corrupt Federation wouldn't give a rats ass about a derelict OYW vintage Pegasus class

Saw vids on the Torrington attack enough to get me very curious about the Pegasus-Class in the video

2:41
*Remains of a highly valuable Pegasus-class Assault Carrier lands onto a mountain on Earth*
Zeon Remnants: "Its free real estate."

I'd say that the Federation wrote it off after it crashed during maneuvers. They have a record of using things far beyond their intended life cycle, so it was probably so outdated by the era of Unicorn that they grabbed the stuff that was marginally sensitive and the things that could be used to possibly reactivate it as a ship and called it good. Cleanup was never a strong point on either side in UC, with remnant bits of every war just hanging out all over the place. There were many instances of whole mobile armors just ignored in civilian hands across several series based in UC. Corruption was very common in both sides as well, so even if the Pegasus-class in question was stripped, Federation bureaucrats could be bribed into leaving bits behind, "forgetting" to log its status, or even listing it as rubble. Not to mention that the black market was very active and it wouldn't be too far a stretch to say it WAS scuttled, but then there was enough old Federation materials to even get the thing back into space under its own power.

I just feel that I have to share the existence of yet another gray-phantom type Pegasus class ship——Forrestal. It appeared in the Manga version of missing links...and was sunk by   An RX-80PR-2.

My theory is that its a downed ship during the OYW especially since that location looks like its from South East Asia which saw heavy fighting between the EF and Zeon. They might be planning to take this one out but because the Titans are far more investment in fighting the AEUG and Karaba which are far more financially supported than the remants as well as Jamitov's influence, they might had just put it asside first and focused on what they saw as a bigger threat.
There's also the fact that the EF neglected their Ground Forces since most battles took place in space with the fact that the Jegan saw most action in Space during its introduction than on Earth.
You can also saw that the EFGF being neglected during the early stages of the OYW as they focused more on building their fleet rather than fortifying the weaker parts of the Earth as well as still using saberfish fighters after the war.

Watching this video I’d say it’s probably an unnamed pegasus class that was taken down on earth because I can’t see a non functioning ship surviving reentry and then crashing while still keeping it’s general form.

I think the "left intentionally there by Feds to encourage remnants" theory very plausible. Seems like a course of action that is in line with the titans and the radicals. After all, (i am not sure where it came from. My last impression was Unicorn, i think.) I recall federation officers creating trouble by LITERALLY funding remnants, and then pointing at those remnants and going "Welp, the government better give us money so we can deal with these pesky remnants!"

During the first Neo Zeon war the few surviving Titans decided to support Haman (i always found this amusing, but i guess they had their back on the wall at that point) and that's why some of their units ended up in their hand. That Pegasus class (which in my opinion was a sister ship of the Gray Phantom and not a ship seen in other previous works) was probably a reactivated mothballed ship operated by a skeleton crew of Titans survivors and a more numerous complement of Neo Zeon and Zeon remnants. While outdated, the Minovsky Craft surely was advantageous on Earth, but it was probably shot down during the conflict. Crucially it's seen launching an heterogeneous group of Marasai and OYW Zaku Cannons. The third theory explains not only why it's still there, but also why so many Zeon remnants cells were still active.

From the MS that leave the ship I would guess it was being used during the First Neo Zeon war. Most likely got ambushed and crash landed into mountain. Why the Feds left it, well the Neo Zeon invasion probably kept them busy. I’m mean heck we have MS break out of a castle that was used as a Neo Zeon base during the invasion, how did the Feds miss that.

I think those remnants attempted to hijack it, but in the process it got too damaged and was forced to crash land. And the Feds likely couldn't find it because of M-particle's deployed during the hijacking.

Is it possible that the ship crashed in such a remote back woods area that federation didn't care?   The federation seems to be pretty nonchalant with spending on new toys. If an outdated piece of hardware crashed out in the middle of the woods,  is it worth spending to recover it when you can spend all that money on building another General Revell.   Another possibility, the Federation doesn't know where it crashed,  there is a lot debris floating around and big chunks of Colony have impacted the earth . So a ship wreck is probably not going to get reported to the authorities.

0:57 That ain't no base its a village too. Its got a little rickety bridge leading to the upper deck and little shanty towns all along it, that's some Guerrilla tactics right there the ghost of Ral is pleased.

If there was a Sinanju and Rozen Zulu on the scene,

The ship and all of those mobile suits would be absolutely f****d

From the angle it's at, it looks more like the ship backed in to the mountain rather than crash while falling. Perhaps the Federation crew was attempting to pull a maneuver like the White Base at the Seattle stadium. Human error then caused it to ground on the mountain while flying backwards. After the crash, the Fed crew abandoned the ship and Zeon remnant forces occupied the downed vessel before it could be scuttled. As an older ship, in a state that it couldn't be made airworthy again, and out in the middle of nowhere the Federation gave up on it rather than sink more resources in the a downed ship.

Judging by the way it's resting and the unpainted sheets of metal covering over obvious damage from an attack and how little sign of visible rust it looks like it was downed around Zeta or Double Zeta series and the battle took place within the Earth's atmosphere.

Considering how chaotic a time that was for the Federation an old ship being sunk somewhere in the middle of Southeast Asia wouldn't be that big of a priority.

Suggestion for a question of the week: How did Zeon remnant groups last on Earth for such a long period of time after the OYW invasion?

They're still out there in UC0096. I only partially buy the argument according to which the Feds want them so they can have an enemy to fight. I think that's true in space, where the Feds are obsessed with projecting their power, but they are completely uninterested in administering Earth, except maybe some regions. So if they're not getting handouts from the Renpo, what's keeping them in the fight? Loyalty to Gihren? To old Deikun? Handouts from the Vist foundation? Just plain old spite?

My guess is the ship was a supply ship that was attacked by Zeon forces while it was flying in the area which lead it to hit the reverse gear in total panic but they didn’t check what’s behind and crashed into the mountain. Since it was under attack, they couldn’t scuttle the ship as much as they needed and probably got wiped out while trying to escape which lead to the ship retaining some of its function enough to be used as a stationary base.