My housemate, not gf (but she IS definitely female), knows where the key is to the gun room and the safe. We also have a pair of digital safe's for easy access that we both have access to. It's not uncommon for her to ask me to get something out of hers and bring it with me when headed out to meet her somewhere. Mine's pretty much empty these days but it's handy for locking up a handgun if a non gun person's coming over or children will be in the house but I don't want to take it to the gun room.
If I was going to be concerned it would be with the amount of access she has to my computers and checking account. However, I trust her in that regard and all of it's easily locked down if needed, whether from her or in general. I agree that it's a potential problem for many folks, but she and I have a lot of history together and there's a lot of trust there that goes deeper than material goods. Certainly there have been live in gf's that did NOT have that access and I have a couple friends, literally one of each gender, who I would give access without reservation if they had any need for it. One's not local, the other is perfectly well equipped and is more likely to be raiding my pantry or fridge for a snack or beer than needing one of my firearms or other resources.