Gate

So what does this spell do?

The short answer is nothing.

The long answer is this was a very old grandfathered spell from stock MERC, in stock MERC it is mob only and it summons/creates a mob (typically a demon) that would assist the mob that summoned it in fighting the player. The Immortals deemed that it was appropriate to have this spell disabled/removed from the game.

So, the question is, does Millament's Staff do anything?

as Explained above nope it does not :/