What kind of information would be pertinent when creating a gang for the Players to be a member of. I've got a few chunks of information noted, but it's all subject to change.

I've got a slight theme, two possibilities for a name, and a leader and bodyguard with basic personalities and names.

What other things should I be coming up with for a gang?

Also, I'm pretty sure one of the books I've got lying around from 3rd or 4th ed has a guide to this stuff, but I can't for the life of me locate it. Anyone got a book/page ref?

I don't recall an out and out "guide" for it OTOH, but New Seattle has writeups on the gangs of Seattle. That should give you plenty to go on as far as what needs to be covered, at least to the point that the canon gangs are covered.

Really though, you've got most of it. Name(s), leader(s), theme. Now you need territory, size, and "what they do" on a regular basis.

I seem to recall some info on running a Gang-style campaign back in SR3's Shadowrun Companion, but I can't be sure.

Yes, definitely the SR companion. back in the alternate campaigns section with doc wagon and lone star...

Some other things you should be coming up with are where is the gang's turf, their size, whether theyre a street gang or go-gang, what affiliations they have, what businesses theyre involved in, and what their tag looks like.

Ahh, a tag. Good call, hadn't thought of that one. It's probably going to be an RFID tag though, since it's a 4th ed game.

Look at the politics within the gang. They'll be relatively undisciplined compared to other groups, so interpersonal loyalties and conflicts will be far more relevent than a more professional group.