Dogmax.... welcome aboard. Photos of your GP-100 are clear enough to show what we call "forcing cone erosion." Technically, "barrel face erosion" might be more accurate, as the cone itself does not erode; or, if it does , erosion is way down the road.
Your forcing cone erosion----with rivulets instead of smooth all around----sometimes indicates a characteristic of the sandblasting effect of large charges of ball powder on stainless steel. The same effect may show somewhat more smoothly on chrome moly steel. Factory .357 mag with 125 JHP erode the cone faster than heavier bullets over a lighter powder charge.
Erosion shown is inconsequential to accuracy. The above mention Lil Gun has achieved the status of NOTORIOUS from some shooters. I haven't the experience with Lil Gun, but it is a question to ask. As you photos show, the GP-100 has plenty meat around the barrel socket, importantly including the six o'clock position. This digression is to compliment you on selecting a strong revolver.
In my loading, I would consider this advanced erosion for 1200 rounds. Which raises the question, what are you shooting?
The 125 JHP is a fine high performance bullet in the .357 mag, capable of excellent accuracy at 100 yards. The 110 JHP much less so. Both tend to more aggressive erosion than the 158 and 180 JHPs.
A forum "diagnosis" is impossible without knowing the ammunition.
David Bradshaw