Here is a zip of everything in my tmp directory after the segmentation fault on my data. And as I mentioned, openmeeg_gain_eeg.mat does not appear in my case:
https://www.dropbox.com/s/pc7iwx8mt062ajk/tmp.zip?dl=1
I tried it with a single thread and the same thing happened.
I did what you suggested with the epilepsy tutorial and it looks like the same commands finish to completion in this case:
pythagoras@pythagoras:~/.brainstorm/openmeeg/linux64$ LD_LIBRARY_PATH=$PWD OMP_NUM_THREADS=1 ./om_gain -EEG ~/.brainstorm/tmp/openmeeg_hminv.mat ~/.brainstorm/tmp/openmeeg_dsm.mat ~/.brainstorm/tmp/openmeeg_h2em.mat ~/.brainstorm/tmp/openmeeg_gain_eeg.mat
./om_gain version 2.4.0 compiled at Jul 13 2018 15:57:20 using OpenMP
Executing using 1 threads.
| ------ ./om_gain
| -EEG
| /home/pythagoras/.brainstorm/tmp/openmeeg_hminv.mat
| /home/pythagoras/.brainstorm/tmp/openmeeg_dsm.mat
| /home/pythagoras/.brainstorm/tmp/openmeeg_h2em.mat
| /home/pythagoras/.brainstorm/tmp/openmeeg_gain_eeg.mat
| -----------------------
Using variable with name : matrix
-------------------------------------------
| Elapsed Time: 2.00185 s.
-------------------------------------------
pythagoras@pythagoras:~/.brainstorm/openmeeg/linux64$