Technically parts of the mesh may be left unsealed, but only for the gbxmodel portion of the object. Collision is what is required to be fully sealed.
Take for example this crude construction of a handgun:

These sets of faces shouldn't create any problems when exporting for a gbxmodel. Note the faces that make up the slide
are intersecting each other, and the faces creating the gun's frame have space between them. This will still produce a functioning gbxmodel.
Collision, on the other hand, may get away with intersections in only one case (that I know of, at least):

Here, the slide and the lower receiver are both intersecting each other. However, they
both are sealed air-tight. This construction of boxes will produce a working collision-geometry tag. The slide can even be linked to its own separate frame so that it can be animated separate from the rest of the model.