Quick answer: GMod won't start: read console log, check workshop addons, map exists and increase RAM (8 GB+ with many addons).
If you landed here: srcds crashes on start, Lua errors in console or restart loop in panel.
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Frequently asked questions (FAQ)
GMod server won't start – first steps?
Read console log. Common: missing workshop collection, Lua error in addon, wrong map, low RAM (min 4–8 GB depending on addons).
Lua error on startup?
Remove last installed addon. Update Garry's Mod + server. Check workshop collection ID in server.cfg.
Which ports for GMod?
27015 UDP/TCP (game) + 27005 UDP (client). Source engine – open both protocols.