The cap is at least 24, although the skillup rate is very low at level 23 (one in 5 or 6 synths). I'm pretty sure the cap is 24. I'd assume the other Earth/Brass Sheet recipes, like the Bastokan Leggings, are also about 23/24.
Mysterytour is wrong. This item caps above 20. I have 21 goldsmith and just got 0.1 skill gain from crafting this. I have gotten skill gain above 20 from leggings and subligar as well -- basically, any CP recombine with brass sheet seems to be ever-so-slightly above 20.