Garry's Mod in two minutes
What GMod is, what Sandbox is, and what Elipcee adds on top.
The base game
Garry's Mod has no story and no levels to finish. It is a toolbox: you place objects, weld them, play with ragdolls and weapons, build machines. Content comes from Half-Life 2 and Workshop addons.
If you have never launched GMod outside a server, start a solo Sandbox first (New Game → Sandbox). You will see the Q menu, the purple physgun and the Tool Gun with nobody explaining them.
Sandbox, DarkRP, TTT…
A “gamemode” sets the rules. Sandbox = free building. DarkRP = jobs and money. TTT = traitors. Elipcee runs Sandbox, with extra rules on top: protected Build, factions, PvE, economy.
You do not install a gamemode. Connecting sends what you need.
What the server adds
Vanilla Sandbox has no factions, no crates, no invincible Build mode. Elipcee keeps building, and adds a game on top when you want it: zones, bosses, shop, auction house.
Server addons (ARC9, PAC3, Glide, etc.) download on connect. Install the Workshop collection so you arrive ready.