[ubuntu] Problem with Huion 610 PRO

It's kind of embarrassing... my graphic tablet seems to be working just fine, but like a mouse, not like a graphic tablet.

In Inkscape, I can draw free hand lines smoothly, could set the pressure axis of the device in the calligraphic mode. However, in GIMP, I can only drag and drop or click on the stuff... no line are draw with the tablet, just with mouse.

xinput shows

⎜   ↳ HUION PenTablet                             id=13    [slave  pointer  (2)]
⎜   ↳ HUION PenTablet                             id=14    [slave  pointer  (2)]
⎜   ↳ HUION PenTablet                             id=15    [slave  pointer  (2)]

Any clues?