If you do not have pen pressure by default in GIMP, please follow the instructions listed below to set up pen pressure.
Open GIMP > Edit > Input Devices
 these three selections need to be set for  Screen
Once you have done that, you will need to close and then restart GIMP
 You will need to set Dynamics to Pressure Size
You may need to stop and then start GIMP once more
The image to the right shows that my Xencelabs Pen now has pressure sensitivity.