Gmsh does not seem to import the group into exported VTK dataset. Therefore the following workflow seems to be 
needed.

In Gmsh, make a seperate .geo file for each body. Then export the grid for each physical volume into VTK format.
Then you load all of these in Paraview, Use Calculator to assign values, and then use AppendDatasets. Then you 
can save as a .vtu file. 

For just the grid, you can save the whole Gmsh grid as a VTK grid. 

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Since we only want \nabla \cdot M on the calculation mesh do the following:

	I.
		1: In Gmsh make .vtk files for Grid_merge 
		2: In Gmsh make .vtk files for BoreholeMagnetPlus (2D grid!) 
			When doing this make sure you save all in the GUI. Or else no cells are exported.     
 
	II. In Paraview
		1. Open Grid_merge.vtk file
		2. Open BoreholeMagnetPlus.vtk file
			->Use Calculator assign 0 to everything
			->Clip with normal
				Use Calculator to assign -1
			-> Clip with -normal 
				Use Calculator to assign 1
			-> Clip of ends. 
		3. cntrl-click the three Calculator datasets. And use AppendDatasets Filter.
		4. Save Data (Select Appended) as .vtu
		5. Disco