How can I go back to GIMP 2.6?

Try doing the forward port approach (works for more packages than just Gimp):

1. Add the precise main repositories to your system.

   Create a file like /etc/apt/sources.list.d/precise-for-gimp.list containing

   deb http://nl.archive.ubuntu.com/ubuntu/ precise main restricted
   deb-src http://nl.archive.ubuntu.com/ubuntu/ precise main restricted
   deb http://nl.archive.ubuntu.com/ubuntu/ precise-updates main restricted
   deb-src http://nl.archive.ubuntu.com/ubuntu/ precise-updates main restricted
   deb http://security.ubuntu.com/ubuntu precise-security main restricted
   deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
   

   Replace the mirror (here nl.archive.ubuntu.com) with the one you would like to use.

2. Add a file with the appropriate pinning to allow downgrading of a certain set of packages, in e.g. /etc/apt/preferences.d/10-pin-gimp-to-precise:

   Package: *gimp*
   Pin: release a=precise-security
   Pin-Priority: 1001
   
   Package: *gimp*
   Pin: release a=precise-updates
   Pin-Priority: 1001
   

3. Run sudo apt-get update

4. Run sudo apt-get -s install gimp (-s to simulate only)

5. Check whether the output makes sense. If not, and it can't manage to satisfy the dependencies, then consider reverting the actions above. If it does work, then actually run the actions to be performed by re-running the previous command without the -s option.

This approach will provide you best flexibility and security-update support, as it's just using the repositories rather than a specific version. It may be a PITA to set up, though.