Does anyone know about SteamID

Uhm, No... The best way to get a steamid is to just put their steam community link into the steamid finder. You could get it from servers on Gmod If they have GmodAdminSuite but other than that Idk.

ulx will tell you your steamid you just install it. or just start a server it will tell you in console.

use the steam id finder people use it all the time its usually in your link too

You can enter status in the game of valve to see you steamid

Status in console when in-game, then you can see your Steam:ID

use this site for Steam ID : https://steamid.xyz/ (that very easy to use)