Hi Avangi
A guest room is a bedroom in a house or hotel for visitors or guests to sleep in. (Collins Cobuild Dictionary)
I think, in AmE, it should be 'guestroom', but in BrE, 'guest room' is also correct.
IMO, it is not wrong to write 'the guest-room window' or would it be better to write 'the window of the guest room'?