Here’s what Channing Tatum has said about the scene in the press.
Warning: This article contains spoilers fromDeadpool & Wolverine.
Though the battle seemed to be going in their favor, the all-devouring being Alioth arrived.

Channing Tatum’s Gambit in secret ‘Deadpool & Wolverine’ scene.Marvel
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But there’s something that’s happening in that moment that maybe no one knows yet."
He then deflected his tease by adding, “You never know.
But [Reynolds] fought for me and Gambit.
I will owe him probably forever.