Dear Mohamed,
Assuming you have a CAD description of this mask, you can use FluDAG, which is an integration of DAGMC within FLUKA. It supports the decomposition of your CAD surfaces into high resolution tessellation primitives. More info on FluDAG at [1], and on its installation here.
The GDML format is used as geometry input description by e.g. Geant4. You cannot use it as an intermediate step into FLUKA here, as complex shapes would not be supported by the conversion process.
Hope this helps,
Cheers,
Gabrielle
[1] Andrew Davis, Janet Barzilla, Alfredo Ferrari, Kerry T. Lee, Vasillis Vlachoudis, Paul P.H. Wilson (2019). FluDAG: A CAD based tool for high energy physics. Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment. Volume 915, Pages 65-74. https://doi.org/10.1016/j.nima.2018.10.117