Here's how you get your tablet to work on GIMP:
Open GIMP
On the upper toolbar, click EDIT -> PREFERENCES
On the lefthand menu, click INPUT DEVICES
Click CONFIGURE EXTENDED INPUT DEVICES
On the lefthand menu, click PEN and change its mode to SCREEN
Click SAVE in the lower righthand corner
Click OK in the lower righthand corner
Restart GIMP
You can now customize your own Dynamics or use the premade ones

. Enjoy your new pen pressure!