Boris FX (formerly GenArts) no longer supports Monsters GT, and it does not work with current versions of Adobe After Effects CC.
To uninstall Monsters GT from your computer:
For Windows, go to Start -> All Programs -> GenArts Monsters GT for After Effects -> Uninstall. Select “Uninstall”.
For Mac, go to /Applications/GenArtsMonsters-ae/Uninstall MonstersGT and double click on “Uninstall Monsters GT”.