Regarding to hex-dominant mesh, please follow my comment at this treads discussion
After finished, check Tool-Statistic in GMSH and be sure there’s only contains hexahedral and tetrahedral element. Nothing else i.e pyramid since it’s not supported by CalculiX CCX solver.
openings in CalculiX GraphiX is good decision, since many further process may required
*Display group of element type
*Merging nodes by smallest tolerances
*Merging collapsible element
*Element mesh quality (Jacobian range numbers)
*Remove unwanted element type 1D and 2D if available
were mostly above features, currently not available in PrePoMax, alternatively someone can use Salome CAD/CAE instead of CGX