Other times the "call a witch's power" potion was used, the potion called for a fresh human heart for each witch's powers a person wanted - for example, earlier in the season when Tawatha did it. "Aunt Gail" didn't have 3 fresh human hearts. Also, when Tawatha used the spell, the Charmed one didn't have to drink a potion. When Aunt Gail did it, she put a potion in iced tea for the girls to drink and lose their powers. In addition to this, Aunt Gail gives the page from the book of shadows to Phoebe for them to reverse the magic and get their powers back from the demon. The sisters didn't have a fresh human heart to include in the potion either. Yet the girls have a potion to give to Cryto.
Everyone says Aunt Gail was desperate to be young again because she was sick, and they mention radiation and chemotherapy treatments. The problem is, cancer isn't an "old person" disease, it attacks people of all ages. If Aunt Gail has cancer, being young again wouldn't necessarily fix her.
When Phoebe is getting directions, she tells the men that the glasses they found aren't hers. But at the end, she shows Prue the same glasses that she left behind earlier.