He is an alternate universe version of Gabriel.
In 2018, when the Main Universe Gabriel encountered Michael, Michael was surprised to see his brother, but appeared pleased. However, he was able to quickly identify that it was not his universe's Gabriel and hence had no qualms fighting and eventually killing him.
Assuming he did survive The Apocalypse, it can be presumed that he would still die in 2020 when God started to destroy the Multiverse.
Gabriel's true form is similar to the main universe's version of the Archangel, as alternate Michael was able to recognize the latter, but he emanates a different aura.
- His history is the same as the Gabriel of the Main Universe up until 1973 when the universes diverged.