Hi! I have muscle memory to press the space bar to get your character to stop walking when firing at monsters but this doesn't seem to work in the GOG version? Does anyone know if there is a way to do this? Thank you!!!

The GOG version, like the original disc version, uses spacebar to dismiss various overlays. There is no need for a key to "stop walking when firing at monsters" because you cannot be both walking and firing at monsters, so you cannot need to stop doing that. A character can be idle, walking, melee-attacking, blocking, firing a bow, casting a spell, doing hit recovery, or a few other actions - but never more than one at once. If you are walking, you are not attacking. If you are attacking, you are not walking. Are you thinking of one of the sequels, which may have had different rules?

That doesn't work in any version. (Perhaps there's a mod that does that, but not the retail version.)

You could, however, use a keyboard remapper utility like AutoHotkey to remap your spacebar to act like the Left Shift key --which is the key that makes you stand in place while attacking.

Hold Shift to attack in place if that is what you mean