How hard is it to install these parts?
These parts are straight replacement parts. "Bolt on's" There can be a little bit of friction due to the anodizing process (colored parts) and will possibly take a little more effort than the stock parts. It is a worthwhile upgrade tho.
What is the difference between gpm parts and stock RS parts?
There are several differences between gpm parts and stock parts.
1) GPM parts are much stronger than the stock plastic parts and will not break under pressure that would snap stock parts.
2) GPM parts come in several colors and stock parts are just one color.
3) They look much better than stock too!