“Don’t drink tap water at Jerry Garcia’s” is a nod to the fact that Jerry Garcia was a big acid dealer/manufacturer. You could distill water and add soluble LSD to create acid water, which the Grateful Dead did pretty often. Gex is basically telling kids not to do drugs.
Gex Fact: I met one of the devs for this game, and the reason why there’s all those random/strange lines is because the game development team were all allowed to submit suggestions for lines they wanted in the game. It could be whatever random nonsense they wanted. ALL of the submissions were accepted, so anything the team could think up got put in!
-Gex