It's not Gordy's music, he just said he never heard this music before.
The brass are definitely recorded, and for the strings it could be real orchestra with a layering of samples on the beginning because of the edit version for the advertising.
Also, yes it's close of the Imperial March, but depending of what happens in the movie it could make sense to have something close of the Imperial March.. So in the end it could be something from the TLJ score.