Funny that Dessix was referenced here.
Anyway, it depends on whether or not the addon uses serverside files
Serverside files cannot be stolen because they're only on the server.
Shared files can be stolen because they're both client & server
Clientside can be stolen because they're...clientside.
The files you want are likely in your cache folder (if they're not serverside) and you can view them, though they're encrypted now (Garry did this after GMod13 update) if you can decrypt them and read them you might be able to use it.
TL;DR Yes, and no, it depends on whether or not the addon relies on the server.