Comment 2 for bug 1827298

The top surface and potentially all other surfaces need to be added as "Physical Surface" to the geometry otherwise the surface elements are not written into the mesh file.

It should work in the follwoing way:

out[] = Extrude { ... } { };

Physical Surface("top") = { out[0] }

PS: The "out[]"- list is explained in https://fossies.org/linux/gmsh/tutorial/t3.geo