I'm based in Canada and I have similar situation for my Serbian property .
You don't have to register for GST if your property is not in Canada.
I got this answer from Airbnb support teem:
Airbnb Support12:40 PM. For the Hosts who don’t provide their GST, HST, and/or QST IDs, Airbnb will charge these taxes on top of their nightly base price for all the listings located in Canada. So since yours, is in Serbia, no need to provide a GST ID.
The email was sent to all Canadian Hosts as system generated but since your listing is located in Serbia, you are not required to provide a GST ID