While I have a girlfriend, can my best friend be female
Would you be comfortable with your girlfriend knowing everything about your relationship with your best friend?
If the answer is no, then that's not a good sign. If your best friend is -- biologically, anyway -- a threat to her status as girlfriend then you need to be absolutely transparent about your intentions and attitudes. If you find your best friend sexually attractive, crush your libido whenever you're near her and don't put yourself in a position where wanting to have your cake and eat it too -- a very human response -- leads to you betraying your girlfriend.
Despite (or because of) your openness, would having such a relationship in the first place feel threatening to your girlfriend?
If so, then you've got a simple decision to make: Your girlfriend, or your best friend? Ask yourself if your girlfriend is being rational, possessive, calm, moody, or oversensitive. Ask yourself if you've been neglecting your girlfriend (in general, or because of time with your best friend). Ask yourself which relationship means more to you. If your girlfriend makes you happier than you've ever been, and there's something about this other friendship that scares her, then you may need to say goodbye to this best friend for a time. Or, if she's being irrationally jealous and should know that she's number one in your book, you may need to say goodbye to her.
What you can't do is lie. What you can't do is hide.
Depending on how serious your romance is, we're talking about trust and building the future here. Part of that process includes sharing significant parts of your life with your Significant Other. Ain't nothing more significant than a best friend.
Yes, gender introduces complications. It may seem unfair that you have to jump through more hoops because your best friend's name is Sally instead of Frank. Girlfriends and boyfriends are always irrational about something. How you choose to deal with it is your business. Just do it honestly.