How to change Gmod's Resolution to 1080P?

On the video settings the maximum resolution I can get is 1050P and it looks a bit distorted and weird, is there any way I can add the option in for 1080?
Thanks.

What’s your monitor’s maximum resolution?

You may have to change the widescreen settings option for it to show up.

If it’s 1920x1080, you’re going to have to set your aspect ratio to 16:9.

launch it with launch options -width 1 -height 1080

you now have 1080 scanlines, even if they are only 1px wide.