It's impossible to do without a backdoor or exploit existing on the server. Serverside files are only run on the server and therefore have to be retrieved and sent to the client (via an exploit/backdoor).
You do, however, get access to any shared files and client files. You can find these in your Garry's Mod game folder, they are encrypted, however. You can use tools like this to decrypt them: https://github.com/handsomematt/glua-cache-extract