@Inna22 I offer a grill at my cottage, and I do provide propane to use it. I leave one bottle hooked up to the barbecue and a second in the shed, which is full in case they run out. If you do decide to offer gas, make sure to let people know not to use a cylinder exchange program, because often you will end up with an old, inferior tank: I tell guests that if they do run out, use the one in the shed, and if that's gone too, then have the cylinder refilled, not exchanged. I have a reminder stuck to the outside of both cylinders.
It would be nice to offer free gas, but I think it's up to you. If you don't, I would just make it clear in the listing that the guest may have to purchase propane if they want to use the barbeque, or set an expectation like if a guest uses the barbeque, they should refill the tank for the next guest.