I think we might have overestimated Grandpa Gohan. He clearly stood no chance against Goku, getting his Kamehameha dodged and his kick tanked. He also admits he was no match for Goku. Vs Tao, Goku at least admits his all out attacks hurt him a bit…
Chapter: 108, P7.3
Grandpa Gohan: “In any case, you really have gotten strong! I never thought I’d be no match for you! Did you receive training from Lord Muten Roshi?”
Goku: “Yup!”
Goku at least braced himself against Gohan's kick, while with Tao he just stood and let him attack him. He also stopped Tao's Dodonpa with his bare hands, but decided to dodge Gohan's Kamehameha. Overall I give the edge to Gohan, although it'll probably be a difficult fight.