Please I would like to know if it is possible or stated in any official resource.
When the Old Kai used his Unlock Ability to released Gohan ultimate form, he actually released his full potential.
Meaning, this is the best that Gohan could get at this time been.
Although it's only my opinion and not an official resource, Gohan could not transform into super saiyan not only because it's a weaker form, also it's because a new individual form entirely.
And that's why he didn't transform into a super saiyan when he reached this level at the first place.