The artists initially had made Gohan a SS during his fight against Beerus*, but the main scriptwriter Yusuke Watanabe (I think that's his name) told them to change him to "Ultimate". That left him with black hair and eyes, but also with a white aura, and in the manga™ "Ultimate" Gohan had never displayed an ability to create an aura around himself (possibly because all his power is already available to him, including whatever the aura would grant).
Gohan only goes SS (for whatever the hell reason) during the ritual in which he lends Goku his power. The problem with Gohan going SS is that back in the Boo arc when Gohan's power unlocking ritual was done and he asked the Elder Kaioshin how can he become this "mightiest warrior", the Elder Kaioshin told him to do something similar to going SS. And with that, Gohan became "Ultimate", which supposedly gives him a SS in base form. So going by this, Gohan shouldn't be able to go SS because in a way, he's already a SS (SS3, even).
*Beerus - "Birusu" was initially a pun on "virus", but Toriyama mistakened it with "Beer" (hence his attendant being named Whis, from "whiskey"), so "Beerus" is a clever way to convey both the puns "beer" and "virus", though I guess one could go with "Birus" or "Beers" as well.