Everything is spot on, exactly what you want
The service, the servers, it's nothing but fantastic. Will help you with whatever it is, even if you're slow like me they help amazingly and keep their cool. Nothing but love for the guy, an amazing soul.
The gamemode matters most. Sandbox and TTT run little server-side code, while DarkRP and custom roleplay gamemodes run a lot of Lua for jobs, shops and entities.
Addons from your Workshop collection add up too, and every player spawns entities and props the server tracks.
The calculator adds those up from data we collected on Garry's Mod servers, then leaves about 15% headroom so the server never runs right at its limit. It's a guide, not a promise, and you can upgrade from the client area at any time.
Quick answers about Garry's Mod server memory.
2 GB runs Sandbox, TTT or Prop Hunt comfortably. DarkRP servers want 4 GB, and heavy roleplay servers with big addon collections need 8 GB.
Yes. DarkRP runs much more server-side Lua for jobs, entities and shops, and players spawn far more props.
Server-side addons do. Maps and models mostly affect what players download, but Lua addons run on the server.
It is an estimate based on data we collected, with headroom added. You can upgrade from the client area at any time.