Is it possible to override a script? For example, I want a portalgun that makes different sounds, so I edit the original script, save my own copy, and pack it in the map, if the answer is yes.

It depends. If the original script is in a VPK, no. Otherwise, yes.

It's not in the VPK. None of the scripts seem to be in the VPK. So, does it just work if I pack the script, or do I need to do something?