I think Arsenal was the one who escaped via boom tube.

This. In the comics, GG is literally a harbinger of Apocalypse (Apokolips?). Not sure if that will transfer over here, but regardless we're supposed to hate him. Spreading misanthropy, paranoia, and discord are literally his superpowers.

The kid you're referring to is El Dorado. We saw him teleporting in one of the trailers from earlier last year. I'm really excited to see him because I'm pretty sure this is his first appearance in anything outside of the old Superfriends cartoon (Dude has never even been in a comic as far as I know).

This series just keeps getting more and more interesting. The finale is shaping up to be nothing short of amazing.

Everything that happened in between season 1 &2 happened off-screen because of a time skip. There's a video game tie in that's supposed to explain what happened in between.