Hi Marc.
I have used GstarCAD with Visual LISP and can confirm that the support is quite good. There are some things that don't work (not necessarily related to Visual LISP). For example, command-s doesn't work in GstarCAD. But the regular 'command' function operates in it's place, where as in ACAD the code would fail.
MOST of my code worked first try. When it didn't work, small tweaks were needed to get the code to work, but it worked... and worked surprisingly well. Much of my code heavily utilized Visual LISP.
EDIT: If you use GstarCAD just for LISP, be aware that the program has some shortcomings compared to ACAD. They may, or may not, have any affect on what you do.