Now that I’m able to use 3.1 from the GUI side of things, I have found a first error.
$ sp cat echo ${rep_test\ 96}
Error Message: [gmic] *** Error in ./*substitute/rep_test/ (file 'C:\Users\User\AppData\Roaming\user.gmic', line #1170) *** Command 'if': Invalid argument '39,61,61,39,112,100,110,95,112,97,108&&whd#-1>96': Unrecognized item '39,61,61,39,112,100,110,95,112,97,108' in expression '39,61,61,39,112,100,110,95,112,97,108&&whd#-1>96'.
[gmic] Command 'if' has the following description:
rep_test:
format_choice=${arg\ $1+1,gpl,pdn_pal,jasc}
use_pdn:='$format_choice'=='pdn_pal'
if $use_pdn&&whd#-1>96 u pdn_mode
else u non_pdn_mode
fi
This used to work by the way. Can be fixed easily though by wrapping with [].