It seems to be necessary to create an element set as well.
Physical Surface(“channel”) = {31};
Then when I export the mesh in gmsh-format version 2 ascii without the include all elements option and import it in Freefem++ then I get the three labels 2, 3, 4. Label 1 is assigned to channel in the msh-file.
By the way in gmsh the elements inherit their numbering scheme (clockwise or counterclockwise) from the underlying geometry. In Freefem++ node numbers have to be given in counterclockwise order (as with most FEM-programs I know) otherwise you get the “assertion error: Area > 0”.
This means for Freefem++ the elements have a negative area. Such elements are also called inside out in some commercial finite element programs.