“And after that ye were blessed” refers to verse 26. Jesus is sent to bless them. His presence fulfills that promise, and therefore they are currently in the condition described by verse 26. The promised blessing will occur after this appearance. The point is important because 1 Nephi 19:10–16 might be interpreted to mean that the gathering was coterminous with this appearance by the Messiah.
“Then fulfilleth the Father the covenant” explains the next step. The Savior will fulfill the covenant promise to Abraham in gathering not only lineal and scattered Israel but all who accept the gospel and are spiritually adopted into Israel (3 Ne. 16:12–13). This clause describes the context for the next phrase: “unto the pouring out of the Holy Ghost through me upon the Gentiles.” The outpouring does not refer to the final gathering but to preceding events. As Jesus had explained on the first day (3 Ne. 16), the future includes a period when Gentiles will persecute the house of Israel. They will have power to persecute because their initial acceptance of the gospel will bring a “blessing upon” them that “shall make them mighty above all, unto the scattering of my people, O house of Israel.”
On the first day, Jesus had described a series of reversals: The gospel would be taken from the Jews and given to the Gentiles, the Gentiles would persecute Israel, then the gospel would return to Israel, and the Triumphant Messiah would lead Israel as the wicked (Gentiles) are destroyed. That is the ultimate reversal. At this point, however, he is describing only the persecution and continued scattering of Israel.