I've downloaded the game three times by now, I've added the permission and I've tried installing it with sh gog_soma_2.6.0.7.sh and ./gog_soma_2.6.0.7.sh, but I only get the error message: Can't open gog_soma_2.6.0.7.sh

MD5= 19e36cfa166932ac58fde94b983bcdb9

to install any *.sh file you need to "chmod +x" the file first like this: alternatively right click file select permissions and then select 'Allow executing file as program'. box if it's available.

32-bit distro? Try using BASH instead of DASH.