Using manylinux wheels allows to fulfill our goal of making Slicer compatible with computers that are bought in the past 5 years. I don’t think we would compromise on this for an incorrectly packaged GPL/commercial library. Also, based on what Jc described above, flipping the cxx11 abi flag might not solve all problems.
I would recommend to get/build gmsh binary and launch it in Slicer in the startup environment instead of trying to install it as a Python package in Slicer’s virtual Python environment.
Or - even better in the long term - use open solutions, such as fTetWild or Cleaver.