According to GT, Goku pretty much becomes immortal through joining with Shenron. (End of GT: Goku flies away with the real Shenron and absorbs the 7 Dragonballs into himself).

But yeah, GT is a lot of bollux.

I could understand Piccolo appearing since I guess Nameks can live a long time. Trunks I forgot is a timetraveller, so that 'makes sense'. Goku is the one who doesn't fit if it's based on the Manga, but if they included GT-Immortal Goku I guess it's feasible. Dragon Ball wasn't very good at flawless plotlines anyway