Apparently on GIMP you can't set the stylus button 3 as an instant color picker. The default is to hold control and click, which is not that much of a hassle, but it's interesting to have this kind of functionality more uniformly activated.
A hacky way of getting an approximation of it is along the line(s) of:
Code:
while true ; do xdotool getactivewindow getwindowname | grep GIMP && xsetwacom set "Wacom Bamboo Pen stylus" Button 3 "key Control button
...