Well, I haven't had this specific problem. Everything else failing, I'd go through the usual troubleshooting:
- In the Control Panel, go to "Add or Remove Programs"/"Programs and Features" (its name depends on Windows version), and uninstall anything related to Wacom.
- Unplug your tablet.
- Restart the computer.
- Open GIMP and the input devices and be sure it's not got anything Wacom left in it.
- Go to wacom.com and download the driver for your Bamboo tablet. Be extra-double-sure you have the right driver for your tablet. For example, I have an old Intuos so I had to go to the legacy driver section.
- Install it. Restart the computer.
- If you're running a version of Windows with tablet input features (any version newer than XP I think?) go to the relevant control panels (Pen & Touch, or Tablet PC settings, or whatever) and turn off all features related to press-and-hold.
Read more here.
- Plug in your Bamboo and open the Wacom Tablet Properties program. Touch your pen to the tablet. Your tablet and your pen should now be listed in the interface. Set up your options how you like.
- Open GIMP and hope you can add your tablet to the input devices again.

If that doesn't work, I've seen mention of some sort of
GIMP user profile and that deleting it sometimes fixes things. Make a backup first!