First off NO, do not buy your girlfriend a knife for a Birthday or Christmas gift if you have actual heartfelt feelings for her, if you’re in the relationship simply for the sex or as a placeholder until something better comes along, then sure a knife is a fine idea, at least you got her something.
I’m going to make a blanket statement here and this certainly doesn’t apply to every female or every situation, but for the most part, I’m fairly certain this will apply to your situation.
You’re girlfriend may very well appreciate a good quality knife she can use for many years, but at the end of the day, it’ll be “just a knife” to her, you’ll be much better off planning out the day to take her “knife shopping” and spending the day with her and having her participate in the process of picking out her new knife, she will value the time spent and the whole process you’ve planed out to get the gift, far more than the gift itself, if you buy a “bro” a Chris Reeves Sebenza he’ll be blown away and cherish the knife because he appreciates the knife for what it is, your girlfriend will probably remember where you where and what you two were doing when you gave her “the knife”.
Granted, I don’t know you or her from Adam or Eve, but I can certainly tell you that, unless she is very rare female that thinks differently than 99% of the other females, you will see zero difference in her reaction if you give her a $9.99 Chinese convenience store “punisher” knife or a $1200 custom made for her work of art, because she will remember the events leading up to getting the gift more than the gift itself, of course I could be way, way off base if she happens to really be into knives, but seeing how the vast majority of males who consider themselves to be “knife guys” couldn’t name 3 main stream knife manufacturers that crank out millions of shitty clones every year, let alone knowing the difference between G-10 or CM154 blade steels, I highly doubt your girlfriend will know or care, she WILL remember where she way, what she was wearing and how the gift was presented far more than the knife itself when she sees it laying in a drawer 10 years from now.