I can see no reason as to why not
On some gearboxes the input shaft is slightly proud of the bell housing face, it may pay to stand the gearbox on a couple of thin pieces of wood
This is the way that I have always stored gearboxes
Edit
If the gearbox is going to be stored for any amount of time, put some grease on the input splines & spigot bearing surface.
This will help to protect from rust