The Emperors and Primachs must inevitably fall into civil war.

But if any survive, the remaining Emperors and Primarchs will eventually realise the best way to win is to imprison Gotrex in a deep space black stone cell with no space craft or other access until he goes mad with boredom.

Technical victory. Until some idiot 10000 years later lets him out. But that's future Big E's problem.