I'm a huge Gunslinger Girl nut - it's probably my single favourite out of all the anime I've seen and manga I've read - so I'm genuinely delighted that Seven Seas have brought it back into the light once more. Living as I do on the other side of the pond, it will probably take some time before the new omnibus starts appearing on store shelves, but rest assured that I'll be snapping it up as soon as it appears.
Hopefully the new release will be creating quite a few more GSG fans, and as those freshly-conditioned cyborgs will be drawn here it felt appropriate to introduce them to Gunslinger Girl's presence elsewhere on the internet, and feed more interest and anticipation for future volumes!
Gunslinger Girl's TVTropes page
Handy for newcomers who would like a quick and accessible primer on the series, although this comes with the usual barrage of cautions about just how spoileriffic these Trope lists can get!
Cyborg Central
As far as I'm aware, this is the main GSG fansite still extant on the web. Forum discussion ranges more widely than GSG specifically - a core of members are real petrolheads, for instance - but the community is close-knit and very enthusiastic. If you're creatively inclined members do offer in-depth commentary on fanart and fanfiction.
Gunslinger Girl Fanfiction.net archive
If the wait to the release of the next omnibus volume is starting to prey on your nerves, hopefully the expanded adventures of the Social Welfare Agency depicted here can help tide you over until then. I know that fanfiction is something that gets widely disparaged, and not without reason (or Full Life Consequences), but there is honestly some coherent and thoughtful stuff to be found here. Prof. Voodoo's "Marisa Chronicles" are a series that I'd reccommend.
GSG Fancomic - Jethro + Monty
I once tried to start up a Gunslinger Girl fancomic myself, which sadly never advanced beyond fifty pages of script and twenty or so pieces of concept art before my artist dropped out with life commitments; without that key part of the equation the project was stillborn (although you can still see the skeleton of the site here if you're so inclined). However, in my absence another excellent endeavour stepped up to the plate: the adventures of Jethro & Monty, a fratello currently pursuing the colourful hijinks of a casino heist with a style that allows for dense text while not sacrificing detail. I think that it's very authentic and well-observed, inserting fantastic cyborgs seamlessly into the wider world.
Yu Aida's Personal Site
If anyone is curious as to what else GSG's mangaka gets up to. He also maintains a separate art-blog, which includes art on subjects beyond GSG - Touhou is a recurring theme.
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Anyway, I hope that GSG fans find all of this interesting, and thanks again to Seven Seas for giving us the opportunity to enjoy such a great comic!