Two powers make up the Power of Grayskull, both from the gods. This may clarify:
The gods' aura (the little bits of their energy left behind, which was raw creation power), through a series of events, became known as the "Power of the Universe"
Eventually through another series of events this Power is captured and winds up contained/bonded with the Sword of He which is brought to Eternia by He-Ro. Eventually through a series of events this power is removed from the sword by Grayskull's Elders and contained in an orb hidden in Castle Grayskull.
Meanwhile, back to the gods...
Before they left the universe and only their aura remained, they hid their magic deep inside Eternia in the shape of a star. The Elders of Eternia had the wisdom to tap into this magic. In the 200X series you see them combine their forms and their wisdom to tap into Eternia's magic with the orb (with the help of the Sorceress creating the now unified "Power of Grayskull".
The orb now contains both the wisdom to tap into the god's hidden magic (star seed) on the inside and a shell compromised of the Power of the Universe (which had been extracted from the Sword of He) into one unified power called "The Power of Grayskull" which is used by/channeled/harmonized by He-Man and She-Ra. Hope that helps.