bzr-gtk is a gtk2 app, but gedit is a gtk3 app. The two APIs cannot coexisit in the same process. This is a GTK limitation. gedit-developer-plugin are already gtk3 as is implied in the earlier comments
The work-around it to use the gtk3 version of bzr-gtk. It is no packaged, so no gtk3 app can use bzr-gtk from Ubuntu's archives. Nor is there a package in a PPA of the gtk3 version. You can install the proper code using the command line by tying:
bzr branch lp:bzr-gtk/gtk3 gtk
Restart gedit and enable the bzr plugin.