DBGT Golu is also stronger than SSJ3 Goku from the Buu saga. Chibi GT Goku was fighting equally with General Rildo, who was stated to be stronger than Majin Buu. Gohan's SSJ4 would therefore be equal to Goku's SSJ4 in DBGT. Besides, after havibng become Ultimate, Gohan can't transform anymore (he tried, but he couldn't). DBGT only messed up, stripped him of Ultimate form, and made him a regular Saiyan again.