Since you failed to go to the Book of Hosea where Paul quoted that from, I guess I'll have to:
Hos 1:4-9
4 And the LORD said unto him, Call his name Jezreel; for yet a little while, and I will avenge the blood of Jezreel upon the house of Jehu, and will cause to cease the kingdom of the house of Israel.
5 And it shall come to pass at that day, that I will break the bow of Israel in the valley of Jezreel.
6 And she conceived again, and bare a daughter. And God said unto him, Call her name Lo-ruhamah: for I will no more have mercy upon the house of Israel; but I will utterly take them away.
7 But I will have mercy upon the house of Judah, and will save them by the LORD their God, and will not save them by bow, nor by sword, nor by battle, by horses, nor by horsemen.
8 Now when she had weaned Lo-ruhamah, she conceived, and bare a son.
9 Then said God, Call his name Lo-ammi: for ye are not My people, and I will not be your God.
KJV
God spoke that about the "house of Israel" ten-tribed "kingdom of israel". That was after He had already split old Israel into two separate kingdoms each with their own king, one for Judah at Jerusalem, and another for the ten tribes at Samaria. The ten-tribed northern kingdom of Israel rebelled against Him first (see 1 Kings 11 thru 2 Kings 17).
So God told His prophet Hosea to take a wife of whoredoms to symbolize how the ten-tribed kingdom of Israel had rebelled against Him. Each child born to Hosea was to be called by a name that also signified their rebellion against Him.
Then God said the house of Israel (ten-tribed northern kingdom) would no more be His people because of their rebellion. He then scattered those ten tribes among the Gentiles as He has warned through Moses in Deut.4 and Deut.28.
He spoke NONE of this about Gentiles.
But in final, God then said this about them when He will cast away their sins and gather them back to Himself:
Hos 1:10-11
10 Yet the number of the children of Israel shall be as the sand of the sea, which cannot be measured nor numbered; and it shall come to pass, that in the place where it was said unto them, "Ye are not My people", there it shall be said unto them, "Ye are the sons of the living God."
11 Then shall the children of Judah and the children of Israel be gathered together, and appoint themselves one head, and they shall come up out of the land: for great shall be the day of Jezreel.
KJV
When all is done for this present world, then God promised He will gather both the children of Judah (three tribes) and the children of Israel (ten tribes) together, with one Head over them (represents Christ Jesus), and they shall come up out of the lands they were scattered back to the lands He promised their fathers (which is written elsewhere, but what He is pointing to here). And that day will be great, the day of Jezreel which is means 'God will sow' and is in regard to the SEED of Israel.