Quick answer: GMod connection issues: port 27015, sv_lan 0, workshop addons and enough startup time (addons load 2–5 min).
If you landed here: Garry's Mod shows "Connection failed", "Server not responding" or hangs on loading.
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Frequently asked questions (FAQ)
Which port for Garry's Mod server?
27015 UDP/TCP (default SRCDS). Open in panel and firewall. Adjust in start.sh for custom port.
GMod "Server is not responding"?
Server still starting (addons take time), port blocked or sv_lan 0 not set.
Workshop addons cause connection issues?
Yes – missing addons on client = download on join. Too many large addons = timeout. Test with fewer addons.