Found out when he came back everyone was on a trip and nowhere to be found.

I'm not sure, but that was an episode of "GOMER PYLE: U.S.M.C." not "TAGS" (although, they showed a clip from the TAGS episode, where he joined the marines. It was part of the opening scene, where he told Andy. It was when Gomer was sitting at Andys desk, and remembering). I know it wasn't the first season of GP though, because it was in color.

I watched that episode of Gomer Pyle just the other day. That episode makes me depressed for some reason! I just hated Gomer being stuck in that bus waving and Andy never sees him.