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Concerning Gorgons, here's the short version from Wikipedia:

Around 700 BC, Hesiod (Theogony, Shield of Heracles) increases the number of Gorgons to three—Stheno (the mighty), Euryale (the far-springer), and Medusa (the queen), and makes them the daughters of the sea-god Phorcys and of Keto.

So it would have to be like the elemental kings where you only get a maximum of two; but Euryale and Stheno are immortal unlike Medusa.

Concerning Grendel, he was immune to weapons, so Beowulf grappled him and ripped his arm off. Grendel ran off to his mother to die from his wound. All we can divine from that is Grendel was humanoid (meaning most or all of his equipment slots!). Grendel's mother did live in a lake, and Beowulf's magical armor protected him from her attacks, so she tried to drown him by dragging him to the bottom. However, no description is given so she could literally be anything.