Thanks, I finally managed to get it to work. Unfortunately, the icon seems to display super small. I tried it a few times with some different icons and the size is always really small. I bought some icons from Iconfinder.
Obviously, the SVG should be able to scale up, but I tried some icons from another plugin by just copying the SVG out using the inspector tool and it seems that the GP block button is making them display smaller. What can I do about this?