This was true, Sirzechs knew that Gilgamesh would never do something like this, his brief encounters with him showed him that.
And also Sirzechs doubted that there was any devil capable of reincarnating Gilgamesh.
Gilgamesh had fully mastered the power of the Heavenly Dragon of Domination, Ddraig and that already put him in a class of his own.
And Sirzechs also felt the power of Gilgamesh when he looked into Gilgamesh's eyes, Sirzechs felt the power of the King of Heroes.
When Sirzechs looked at him he felt the magical power of Gilgamesh, it was like looking into a gigantic abyss that swallowed everything in front of him, it was surely one of the most terrifying experiences of his life.
And because of this Sirzechs knew that it would not be possible to reincarnate him as a devil.
Only a piece of mutant Queen could reincarnate him, but even Serafall and Falbium didn't have the power to do that.
