No, it doesn't work, it's there from the SP storyline when Trent and crew jump to the Alaska System to do something there (don't want to put a spoiler here). It's completely non functional in the MP mode.

One rumor that I've heard on how to get in is to line up with its entrance line (Straight line) and you should find a path through. Not sure if that's true though, I haven't been in Zone 21 except in the SP game.

Gate is locked by default.

Yes, you are right. If you line up with the front of the jump gate you will find a hole through. Not worth your time... There are a couple of weapons platforms next to the gate that won't fire (unless you were bad to Liberty). The gate gives you an access denied. It is kinda fun watching the outcasts flying in the area. They just fly their path right into the field and boom! A cheap way to get cargo too.