If the user can’t download the map to their machine they can’t play on your server. If you encrypt the map, it has to be decrypted before it can be loaded, so the user’s machine has to receive the decryption keys. There is no bulletproof way to prevent someone from getting a copy of your map unless you just never use it anywhere except your own personal machine in single-player.
I’m not an expert in obfuscating Source maps to make decompiling more difficult, and you may have a chance there, but you simply can’t put an encrypted map on your server without the method of decryption being included, because otherwise you don’t have a Gmod map, you have an encrypted blob of pseudorandom noise and Source doesn’t know what to do with garbage files.