Tell users that tablet support requires Wintab drivers on the Microsoft Windows platforms
This seemed to be a bug for a long time BUT I think I figured it out.
Wacom AES pens work with Gimp right out of the box, but MPP pens don't. You have to download Wintab to get any pressure sensitivity with a touch screen computer. I used the last one on the list at https://www.microsoft.com/en-us/download/details.aspx?id=49498.
I've spent months trying to learn directx and how to enumerate a new HID device and junk, and I was about to switch to autodesk sketchbook, but it turns out all I had to do was download wintab. You should tell people about this!