A bit of a bump as it's related and i didn't wanted to create a new thread just for it.
As i recently acquired a Huion tablet that was very cheap for its very good quality, while Krita or Photoshop had no problem with it, as i was using Gimp 2.8.20 , i ran into the same problem as probably anyone using this kind of tablet has been running into with that version of gimp.
Basically while the pen and tablet have no problem to move the cursors or select options within gimp, there's no drawing done at all.
I found out that in Edit -> Configure Input Device , for the 3 huion listed entries :
Huion tablet erasure
Huion tablet pressure stylus
Huion tablet puck
The "Mode" at those 3 entries was set to "Disabled" , after setting them to "Screen" like the "Core Pointer" is set to , i finally could draw in this version of Gimp.
Now other problem : it looks like how Gimp manage the pressure with that tablet is really bad, as by example to obtain a +/- max opacity draw i nearly had to near-impale my tablet , as i was valuing a bit my tablet life i decided to change the responsible setting instead
So to solve the problem, i had to go back to Edit -> Configure Input Device and at the Huion Tablet pressure stylus entry, at the "Pressure" setting, i tweaked the pressure graphic curve so i wouldn't have to press my stylus that hard.
So for people with Huion having trouble with their good tablet in gimp and still want to use gimp 2.8.20 instead of going back to an older version of gimp or ditch it completely and move exclusively to krita or photoshop, it's worth a try as it worked for me.