I did bits of the GS myself and also the K12 using a kit of 3M ready cut parts. It was good but not brilliant. The guy who owned my K13 from new did it himself and that was an excellent job, much better than my effort.
If you have the time it is worth a go but not if you are a ham-fisted so and so.
Good to know there are professionals who will do bikes, many won't as they are too fiddly.