I am going to keep this short and sweet but the question has been asked several times by several people to me so if this page helps anyone quickly figure out how to temporarily close the interface for better videos and screenshots in Guild Wars 2, then so be it.
To hide the User Interface for Guild Wars 2, hit Control + Shift + H (then use Print Screen to actually take the screen shot)
However, if you wish to just take a screenshot without the UI, you can use the shortcut Control + Shift + PrintScreen to easily take the picture in Guild Wars 2 without the user interface getting in the way of the beauty of the game.
I normally try and hide the user interface in the intros or outros of videos or when showing an area in videos, so it is a very handy command to know in GW2. If you have any questions, feel free to leave a comment below and I will try to answer or create an article about it.
Thanks for reading!