Emily, I don't think I have anything particularly close to draw on for ideas, but I'll try to wing it. If it's useless for you, oh well. If there's something useful, great.
One line of thought goes back to the 'counters' I mentioned in a different topic. To apply them, though, first you have to slow down your thought processes some (like in leisure at home) and see if there are any comments you're making to yourself that help escalate your anxiety when you're in that elevator with several GG (whatever they are exactly). For one situation I had (very different) the mere fact of slowing down and examining what my thoughts were was enough to make for improvement.
Once you have some of those negative thoughts in hand, start thinking about what you can tell yourself to counter the negatives. This probably isn't the fundamental thought, but for illustration:
Negative: "Girls were mean to me in grade school and middle school." and the anxiety comes from the follow-on "And they'll do it again today."
Counter: "I'm a much more solid and mature person now, and so are they." With the follow-on "So they won't act like that any more." and "Even if they did, they don't have that kind of power over mature me."
I don't know how true those are, and you definitely have to say things to yourself that you believe are true. And if your situation for 'mean to me' is more in line with PTSD, I have no clue what to do.
Assuming it isn't a PTSD situation for you, a different approach that you might also try is something of a divide and conquer. Namely, turn take the anxiety thought and follow it down slowly and rationally. Fine, "they'll be mean to me (today)." What mean things, exactly, are these adult women going to do? -- tell you you can't play Barbie with them? (as we'll say happened in middle school) Ok, so how bad is it, to you today as a mature woman, that snots won't let you play Barbie with them? Repeat through the perhaps many different examples you're concerned with. Some might not be as easy to divide down to something you can eventually relax about the possibility. But I'd hope that knocking down some of them will help lower your stress levels on that elevator.
Related line (my wife is learning, after only 8 years konwing me, not to ask me 'what's the worst that can happen', though it can work for other people; my imagination is way to powerful) is perhaps to consider -- How long is that elevator ride anyhow? What can they really do (again, not one to go to if you're PTSD near this) in that short a time that would really matter to me?
