Abraham was in possession of the records going back to Adam. All the records of “the fathers” from the beginning came down to him. (Abr. 1: 31.) This would have gone back to the time of Adam. (Moses 6: 5.) It would also have included the record kept by Enoch which contained a prophecy of all things from the beginning to the end of the world. (D&C 107: 53-57.) These are the records he studied to increase his own desire to be a man of greater understanding and to follow greater righteousness and also to possess the singular form of High Priesthood known to the Patriarchs. (Abr. 1: 2.)
It is a mistake to assume Abraham had less of the Gospel than do we. He had more. We have not yet risen to his level of understanding or priesthood. I reject the idea that Abraham’s “Gospel” and priesthood was inferior to ours. He was a peer of Adam, Enoch and Noah in his priesthood and the understanding given to him.
Further, the Lord personally ministered to Abraham and conferred priesthood, sonship, and an everlasting inheritance upon him. (Abr. 1: 17-19.)
I think it is a mistake to believe we have more, or even as much, as Abraham did. Reading his record (which is his endowment) it becomes apparent there is an understanding of the heavens, including a detailed account of the path back to God’s presence through the stars, which has yet to be restored to us.