Launch the game, set the resolution and the video settings according to your preferences, then minimise the game (do not close it) and go to the installation path (ex: D:\Games\Steam Library\SteamApps\common\Counter-Strike Global Offensive). Now go to the "csgo" folder, then the "cfg" folder, and there search for the video.txt file (it's the penultimate file). Right click on it, select "Properties", then tick the "Read-only" box. Select "Apply", then "OK" and that's it!
P.S. - If you ever want to change the resolution or the video settings, un-tick the "Read-only" box, launch the game, change the settings to whatever you wish, minimise the game, then tick back the "Read-only" box.
P.S. - If you ever want to change the resolution or the video settings, un-tick the "Read-only" box, launch the game, change the settings to whatever you wish, minimise the game, then tick back the "Read-only" box.