I try to simulate the air flow in a greenhouse. I put the geometry of the greenhouse in a larger domain. My question is the type of boundary condition for the upper part of the domain because if I don't define it, gambit treat it as a wall. Thank you very much.

If I understood your problem correctly you want to model a kind of an atmospheric flow. A good way to model the top of such a problem is the symmetry bc if fluxes through the top are negligible. Otherwise you have to use a pressure boundary (with the correct hydrostatic pressure if you are interested in stratification. If I remember it right, Fluent does calculate a pressure stratification only by setting p_ref=0. Otherwise it is not possible to postprocess the p-hyd_static)