Monsters that are discarded do not count towards being the last Monster in the room; they are treated as simply never having been there at all. If you defeat one or more Monsters in a room, and then any or all remaining Monsters are discarded by any effect, you are considered to have defeated the last Monster in the room that was available to battle, and may pick up your 2 Loot for defeating the last Monster. Therefore, yes, you would be able to use Gimme Gimme GIMME after killing Xykon by virtue of having beat the last Monster that you could battle on that turn in that room.
Note that this principle extends to any effect that discards Monsters...If you are in a room with 2 Monsters, and you beat the first one in battle but used the Muskrat 3000 Loot card effect to discard the second one, you still defeated the last Monster in battle in that room and can pick up 2 Loot that turn. However, if you are in a room with 1 Monster and use the Muskrat 3000 to discard it, you didn't defeat ANY Monsters at all, and do not qualify to pick up 2 Loot.