I have to second Anneintherain here. I would feel very weird if one of my husbands girlfriends would give me a personal gift.
If gift-giving is important to you you might consider giving them something they can share, like a nice bottle of wine or some home made cookies or something. This I have experienced and enjoyed (and have done it myself for BF and his partner).
Cleo - forties straight female
Ren - husband of 20 + years
Bo - BF of 2 + years