Welcome to the OB. You'll get lots of advice. Pick and choose the real world advise you get here w/ your other research and you should figure out will work best for you and the gf.
It wud be used for me shooting around w the boys and for gf on days when I am not home for her protection as well.
The "rate limiting step" here is youe gf using the gun. Given the fact that she's going to be using the gun, I'd suggest a full size, non lightweight revolver in 38 Spec. A lot of other guns have already been mentioned, but I don't think most really apply.
I like lightweight snubbies for concealed carry (and carry one myself), but don't recommend them for a home defense gun. A lightweight snubnose will have one less round,be more difficult to shoot and have more recoil compared to a full size, steel revolver.
I don't recommend a semi-auto, either pocket sized or full sized. Your gf may be the exception, but lots of women find it difficult to rack the slide of a semi-auto. If you take her shooting and she handles a 9mm semi-auto, great. Then I could recommend a double action/striker fired semi-auto like Glock or Smith&Wesson M&P, or possibly a Ruger SR9 if you don't mind an external safety.
38 Special ammo is widely available. Low pressure rounds are cheaper and easier to handle and shoot than the high pressure +P rounds. But when it comes to self-defense, the +P rounds are well respected fight stoppers.
Having said all that, the single best home defense gun is actually a shotgun, preferably a 12 gauge, but the 20 gauge is also very good. Again, in consideration of your gf, for a home defense shotgun I'd recommend an easier to handle youth model.
Here are some nice police trade in 38 Specials at a great price. LINK
ETA: You could also get a full size 357 Magnum and load it w/ 38 Specials for home defense. If at some point your gf becomes comfortable w/ 357 Mag loads, you could then stoke it up w/ them. She probably won't like you if you start her on full power 357 Mag loads. The 357 Mag is a fine self defense caliber.
This post was edited on 10/14 at 1:08 am