I found an intresting code (stlToGmsh) that takes an .stl file and makes a cartesian grid that you can open with gmsh but it is not able to create groups.
Unfortunatly I'm not a programmer (I don't know nothing about it!) so I have some problems in using it.
If someone is intresting here the link:
I have solved the problems of the cartesian mesh using snappyhexmesh.
Changing some parameters it is possible to obtain a purely Cartesian mesh (cubic) or a mesh that snaps to the geometry using only hexa element (the mesher is fast and quite robust).