There are plenty of other topics in the forum discussing how to import boundary markers. See for instance: Transitioning from mesh.xml to mesh.xdmf, from dolfin-convert to meshio - #3 by dokken
Mesh generation and conversion with GMSH and PYGMSH | Jørgen S. Dokken
Converting simple 2D mesh from gmsh to dolfin - #4 by dokken
Create mesh with gmsh, convert it and import it in dolfin, problems - #2 by dokken
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