Gimp and Wacom Issue
TL;DR: if your tablet works just fine in other software, and you can move the cursor around with the stylus under Windows, use the tablet to open Gimp, making sure it stays active during Gimp start. Bambo tablets have a blue light the indicates that the stylus is active, keep that blue light on during Gimp start.
The workaround and tips I will share here assume that you're on a Windows 10 machine, with a Wacom Bamboo tablet. It may be displayed as Intuos PT M under Wacom software or Windows Device Manager.
Many people is having issues with Wacom tablets, specially the Bamboo ones and similar, with Gimp under Windows platforms. The problem is that Gimp doesn't seem to recognize the tablet when plugged, and while it may work as simple cursor, pressure sensitive is not available in Gimp. I've recently discovered a very dumb workaround for this problem and I will share it with you, along a few more notes you can check to make yours work hopefully.
To begin troubleshooting, you can simply connect your tablet and make sure Windows detects it. If you have other software that may use pressure sensitive that you know always works you can also check it works there as expected. In my case I also use Blender, and I have never had an issue with the tablet on it, it just works. If the tablet doesn't work even on Windows itself, you may have to do the standard procedure for every problem in Windows:
- Update Windows
- Update Wacom drivers
- Check Device Manager to see if it appears there
- Reboot Windows
- Reinstall Drivers
- Reinstall Windws
- Etc
Of course, that is the generic 'solution' to anything in Windows. However, if your tablet works fine on Windows, that is, you can move the cursor with the stylus and do things, you don't have to do all that, though, driver updates are always a good thing to do.
When you connect your tablet to the USB port, make sure that the indicator light stays on. If it blinks a few times and goes away, and the stylus doesn't repond at all, keep the tablet connected and reboot the PC, that will force a reinitialization of the driver which may have crashed in the background.
With that out of the way, let's move to Gimp itself. With the tablet conected and working in Windows, open Gimp. In order to verify that Gimp actually detected the device, open the Windows menu, go to Dockeable Dialogs > Device List. It should open a small panel with a list of devices. If you only see Core Pointer in there, Gimp failed to detect the device, even though you may be able to use it as a cursor in it.
Now, here's the little trick. The thing with Gimp is that it tries to load active devices when it starts, and once it started, there is no way to retry that process. It is only done during Gimp startup. The Bambo tablets have an active indicator that lights up when the stylus is near its surface, and it's when you can move the cursor with the stylus. What I noticed, is that if you open Gimp, using the tablet itself instead of your mouse, Gimp will actually detect it and everything will work as expected, but when you have the stylus away, Gimp may not see the device. (Weird).
Once you open up Gimp using the tablet as cursor, instead of your mouse, you can go to the Device List under the Dockeable Dialogs option in the Windows menu. If everything went right, it will display the Stylus and Eraser as input devices. You can then configure them from Edit > Input Devices.
Finally, be sure to actually attach some behaviour to the pressure in order to use it. You can do that from the Tool Options dialog when you select the Brush tool or similar. The option is called Dynamics and it lets you adde some behaviour to the pressure of the stylus/eraser, like brush size, opacity, random, etc. It's easily overlooked and without them active it may look like that pressure is not working.
Gimp has this bug for some time now, and it actually states that in their FAQ (link below). From what I could gather, it has something to do with GTK 2 libraries. Gimp developers will be moving to GTK 3 on some point, expecting this and other bugs get fixed. I don't know when that will happen though. So far the Gimp version 2.10 (in my case 2.10.22), still has this bug, and the developer builds are on 2.99, which might not be stable enough for some poeple, I haven't tried that one though.
Linux based machines apparently don't suffer from this problem, so if you don't mind switching OS, you may want to try Ubuntu, or some other distribution of your choice. So far the GTK issue seems to be present on both Windows and Mac machines.
Hope this helps. Happy drawing!