Does your Ex know the layout of his GF parent's house?
If not, suggest he take the stairgate, just in case, if he does know the layout and is happy with everything, you are not really in any position to insist. He HAS to take responsibility for that. I would suggest a hall light to be left on too. as in, "we had problems at my sister's house, she may need a hall light left on". Then leave it at that.
You can't insist on an adult sleeping with her, that is not your call.
IF she goes and screams the house down, that will be what her father has to deal with. It's HIS watch. TELLING him he has to share a room with her in his GF's parent's house is way beyond your remit here. Does your ExH tell YOU how to parent his DD? Did HE have any helpful hints for you at your sister's house?
Even if you DID attempt to put your foot down, he can completely disregard your instructions. You'll probably never know. By insisting on this, you will come across as overbearing, over protective and frankly a bit of a loon. Sorry, but you will.
If your DD finds it hard to be in a strange house with her dad, her dad can deal with it. If that means she ends up sleeping in with him, (and probably his GF), then so be it. Or are you suggesting this to try to STOP your ExH sleeping in with his GF??? cos that will be how it'll come across.
...The other option of course is that she sleep in with the GF parents, would that be acceptable to you?
This is his call, i sympathise with you, I really do. I'm lucky in that my Ex is out of the country and we have NO danger of having to see him for at least another YEAR or so, but if your current shoe were on my foot, I'd struggle.