And is that perfume called “Intimate Excretions”?
it’s a weird post from anyone. A workplace where people aren’t allowed to close their own office doors while working? Waiting briefly in your SO’s office for him to finish work so you can drive home together means you’re a slut (door closed or open)? Even in the 1950s this post would be strange.
If the OP doesn’t work there, why is she getting all up in the office and/or its politics?
IMHO: If You don’t work there, then you shouldn’t be in employee areas.
Hotels have guest lounges. Take a seat, have a coffee, read the paper, have a muffin.
If you feel the need to get loud and say something that will affect your BF’s job, then by all means, jam in TWO muffins. HARD.
Don’t make anyone you are dating have choose between you and their career.
Ouch, sorry. I wasn’t thinking about that when I wrote the post. 
If there is some policy about closing office doors, then the stuff other than closing the door doesn’t matter. It sounds like the GM verified that he didn’t break that policy, then was done with the BF. You don’t know what the GM did with the lying girl, generally any disciplinary proceedings are considered private, so he wouldn’t talk about it if he did giver her some kind of verbal or written warning. Your assertions about his sexism don’t have any hard evidence backing them, and his being sexist is OK as long as he doesn’t treat people in a sexist manner. The employee didn’t call OP a slut, she reported some observations to her manager.
So if you (OP) call corporate, what are you going to report exactly? According to what you said, she didn’t say anything to you, and didn’t say anything bad to the GM, she just told him the circumstances. The GM has broad discretion on how to deal with her ‘lying’ (which she will claim was a mistake by the time she’s talking to anyone else) and didn’t do anything but ask your BF about the allegations, which is his job. All your call can do is get your BF in trouble, since no one else did anything wrong that wasn’t handled, and him bringing you into the office is probably technically against policy.