It does seem like Earth from any alternate reality is a magnet for trouble.
It should be. Check one of the major news networks or could always google NYC and alien invasions.
We've only ever known them as the Ancients. But their race found a way to turn into pure energy, or ascend so they aren't exactly gone but the majority of them refuse to interfere with what they call lower beings, aka anyone who isn't ascended. But their race is the one that built the Stargates and put them not just in the Milky Way Galaxy but also other galaxies. And even though I've been studying the Gate for over ten years I still don't know everything about it or how it's able to establish a stable wormhole. I do know though that there's an element in them called naquadah.
But the Gate on Earth was discovered back in 1928 or so at a dig in Egypt. But it's fascinating how it works given you need six spacial points of the planet you want to visit plus the glyph of the planet you're on.
I doubt they are, especially if other parties did earlier depending on how long the cube has been on Earth.