Fedora builds of upstream GIMP branch gtk3-port, for what will become gimp version 3. This version of gimp is highly experimental.
download the repo file for your version of Fedora to the /etc/yum.repos.d/ directory with a command like:
sudo curl -o /etc/yum.repos.d/ryanlerch-gimp-gtk3-port.repo \ http://copr.fedoraproject.org/coprs/ryanlerch/gimp-gtk3-port/repo/fedora-20-i386/
then update gimp with
sudo yum update gimp
The following unofficial repositories are provided as-is by owner of this project. Contact the owner directly for bugs or issues (IE: not bugzilla).