I 26M lied to my 26F girlfriend and I'm not sure how to fix it
As the title says I lied to my girlfriend about the dumbest thing ever and I want to somehow fix it but I'm not sure how. This story starts off with me making a friend at university 22F. I was telling my gf about something that happened at school and mentioned my friend in the story with a very obvious female name. My girlfriend said "oh is that a girl" I panicked and said no it's a dude which is such a dumb lie. It's been a month and every time talk about my stories at university I slide around the issue and use no genders by saying they. How do I fix this? I feel really guilty for the lie I told and I know it's stupid but I really don't want to lie to my girlfriend anymore. Is there a way I can do it without her getting upset or suspicious of me?
Side note: this friend in question is really just a friend. I made sure to mention my girlfriend every time I'm with them like brining up something that happened and even have a picture of me and my girlfriend on the back of my phone case so l've set hard boundaries