So if you want to record gmod or take screen shots with 3rd party software, or just be able to escape the ineveitable freezing of the game with alt tab you will need to set a few launch options in steam.
First off open up steam and head over to your games library you will want to select Garry's mod right click and then left click Properties.
Once you click properties a new tab will open, once this opens you are going to want to click set launch options.
Now that you've clicked set launch options a box with an open text field will appear copy and paste the following in, which should look like this.
Click ok and the next time you launch Garry's mod you will be playing in windowed borderless!