The old Kaioshin powered up Gohan to the very limit of his potential. Gohan, at that point became as strong as he could ever possibly become. He doesn't go super saiyan anymore because the physical changes it normally causes wouldn't increase his power, since it can't go up anymore. He's a level 20 Saiyan.
Since Gohan has more potential, Gohan at the end of DBZ is the single strongest non-fused character in the manga. Vegetto is the only one stronger, and that's because he has the inborn natural talent of Vegeta plus the hard-won trained battle experience and strength of Goku, plus the power of a potara fusion AND the power of super-saiyan 2