Colorsets can take multiple options, So Colorset 100 Pixmap ..., VGradient ..., bg ..., fg ..., etc. But since all a gradient does is create a Pixmap for the background, it probably won’t work well with pixmaps, but Tint will as you found out.
Also you shouldn’t be using such high numbers for your colorset, though not that noticiable on modern hardware, if you use Colorset 100, fvwm will reserve memory for all colorsets 0-100, even if they aren’t being used. So you should mostly just define them in order.