Gohan costume (child)

[ALEX_PRIMAL]

He doesn't have the Gohan Costume (child) when he trains with Piccolo, he uses Goku's sword and clothes in his first training.

[Kashra Fall]

I'd say that's because you don't play as that version of gohan. That version is a younger variant and barely knew how to throw a beam. He out grew those clothes when he actually went to train with picollo.