with the release of StaMPS 4.1 you no longer need mt_prep_gamma_snap, you can directly use mt_prep_snap
Explanation here: StaMPS processing error
Just make sure to enter the correct directories for snaphu, triangle, stamps and matlab, we cannot help you finding these. Afterwards, source the file StaMPS_config, so the files and executables inside these directories become available to the shell.
Some report using StaMPS_config.tcsh (instead of .bash) worked better (How to prepare Sentinel-1 images stack for PSI/SBAS in SNAP)