In my short time as a Gamo collector, I started out with two piece mounts on my S1K. I have found that 1 piece mounts work best for me on all my Gamo's. I use the mounts that have a steel screw that screw down into the lock detent on the dove tail.
In the case of my S1K, I actually had to drill a shallow pilot hole in the dove tail to facilitate the set screw, because the S1K has the dove tail built into the barrel.
Gene