No you do not need to be in a faction to get that ID. You can get the ID of ANY faction you want (Except exclusives like Colonial Remnant and SCRA) without joining any faction.
If you want GMG guns, you make a character. You build him up to full rep with GMG, then you can go buy the ID and roleplay all you want as a GMG guy and whatnot bearing Skyblasts and a Karasu.
I didn't quite get the 2nd part of your post. I am assuming that the GB logged off. After pirating you. Probably a metagamer, saw your ship coming, switches to pirate, pirates you, docks, logs off, returns to whatever he/she was doing.
Metagaming is not against the rules but general frowned upon.
Oh and no pirate would enjoy being chased. I mean seriously, if you just robbed a bank, you would not want the police on your tail. You would prefer to leave scott free without anybody in pursuit.
![[Image: PKNeayo.png]](http://i.imgur.com/PKNeayo.png)