As a Canadian, if I don't think too hard, my intuition says: They were standing under the gazebo. Her car is in the carport. Both would be fine, but I think that the gazebo's main function is to shelter us from the sun/rain (since there are no proper walls), so it makes sense for it to be "under." But either way for both of your sentences is fine.

All prepositions are correct, but they have different meanings. It is impossible for anyone to tell you what to say. Say the one that has the meaning that YOU want.