I think you're a bit confused here. In the series proper, his power up from the Elder Kaioshin becomes his new base form. He doesn't transform into it every time (except Movie 13, where they screwed up), just the once and then its always active afterward. When he becomes Ultimate, his hair becomes spikier and the lines on his eyes are enclosed, instead of open like Goku's.
Never is it stated anywhere in the series/guides/whatever that he lost the power, it's just useless fan speculation. And for evidence to the contrary, Gohan still has the Ultimate hair and eye traits for his base form in GT, so the power-up is still in effect. Meaning his SS transformation is stacked on in addition to his Ultimate power, not a replacement branch transformation.