Avoid third-party websites offering a "GMod 13 standalone download" — those are likely outdated, unsafe, or pirated copies. The only safe, up-to-date, and multiplayer-compatible version is the official Steam release.
Once you own the game, follow these instructions to perform a legitimate : gmod 13 download
Disclaimer: Steam depots and manifests change over time. The manifest IDs provided (76561197996963421, etc.) were accurate at the time of writing for the GMod 13 branch. Always verify your Steam console output for errors. Avoid third-party websites offering a "GMod 13 standalone