So I was looking if it was possible to remove the scratch and found this post, where someone said it was possible to remove the scratch using Silver Polish

http://goprouser.freeforums.org/fast-easy-and-inexpensive-case-lens-cover-scr...

So I tested it in this video, but it didn't work and I also might have rubbed off the infrared coating on the lens.

Anyway, I'm going to order the lens replacement kit http://gopro.com/camera-accessories/lens-replacement-kit/

And I will be more carefull with the camera.