No, there were several dispensations before THAT ONE.
The fact that you agree that the Gentiles were not included in the church from Adam to Christ is evidence that you are a dispensationalist.
Like it or not every Christian is a dispensationalist. God worked differently at different times with different peoples.
I don't believe will you find any writings prior to the 1800s that would state God works differently at different times with different people. God works exactly the same way in exactly the same manner with each person throughout time. He saved you and I exactly the same way He saved Abraham and Paul; through His grace by the faith He has given us. There has always been only ONE chosen people of God.