In GIMP (for I bought it) must be under under Berabeiten | Input Devices which enable four different Intuos range devices in which they are set to "screen". That's it. once and for all. From now on bar width and opacity can be defined by pressure and speed. just because. All GNU / Linux versions from mid / Ende2013.
If your distribution still use a kernel or a graphical user interface without Wacom patch, simply search for "wacom Linux Project" (without quotes)