I have some meshes that I bought through the marketplace.
Can I use a floor mesh as a ceiling or wall mesh?
If I drop an item/weapon, will it fall to the floor mesh (now the ceiling)?
Is floor/wall/ceiling just names for a mesh and gravity will work like it should?
Oh, that totally depends on the license agreement with the content creator.
Some artists charge a premium fee for floor meshes used as ceiling.
Also export restrictions apply as those meshes are considered “dual use” 
Well, Im just joking of course 
You can use meshes for whatever purpose you see fit.
Just scale the floor down to tray size and let the character serve a cool drink on it.
The gravity field is unrelated to mesh locations and their name and always faces “down”.
(If you want to see things falling up , you would need to rotate the camera upside down instead).
Thanks.
I had an idea for the starting point in my first level/room and a ceiling mesh would be better suited as a wall.
I also just realized that if you double click a mesh in the content browser, you get a better view of the item where you can inspect it before deciding if it’s the part you want to use.