BASH_SOURCE is not defined in plain sh, at least in Ubuntu
Remove unportable path

Gambit may be installed in a number of other locations.  For example,
if built with --enable-multiple-versions, gsi will be at
/usr/local/Gambit-C/current/bin.  I think it's more robust to just
expect or require that the gsi executable be in the user's PATH when
he runs sense.