For various usages, it is necessary to install external game content like CS:S or Half-Life 2 and correctly indicate the configuration of Garry's Mod the location of files it must use.
Install CS: S content on your GMod server
On your server hosted on mTxServ, you can configure game contents loaded on your server directly in the control panel, in one click!
Go to your manager in the Game Content module and select the content you want to activate on your server: