I would say the new nation bearing fruit is NOT JUST Ephraim with gentiles, but also those from Judah and Benjamin who have put their faith in Christ. Paul quotes Isaiah 54:1 as being fulfilled in regards to the free children of the New covenant who belong to the heavenly Jerusalem, while the slave children of the old covenant belong to 1st century earthly Jerusalem. Paul was from the tribe of Benjamin and he states the new Jerusalem is "our mother". This new nation includes more than Ephraim.
Please read down a little further in Isaiah 54 and Isaiah relates specifically whom he is writing about.
For thy Maker is thine husband; the LORD of hosts is his name; and thy Redeemer the Holy One of Israel; The God of the whole earth shall he be called. For the LORD hath called thee as a woman forsaken and grieved in spirit, and a wife of youth, when thou wast refused, saith thy God. For a small moment have I forsaken thee; but with great mercies will I gather thee. In a little wrath I hid my face from thee for a moment; but with everlasting kindness will I have mercy on thee, saith the LORD thy Redeemer. (Isaiah 54:5-8)
Isaiah is writing about Ephraim, the ten tribes that the Lord divorced, refused as a wife of youth.
And I saw, when for all the causes whereby backsliding Israel committed adultery I had put her away, and given her a bill of divorce; yet her treacherous sister Judah feared not, but went and played the harlot also. (Jeremiah 3:8)
Plead with your mother, plead: for she is not my wife, neither am I her husband: let her therefore put away her whoredoms out of her sight, and her adulteries from between her breasts. Hosea 2:2
Hosea is speaking specifically about Ephraim, the ten northern tribes. Yet, Ephraim finds mercy in the wilderness and the Lord betroths her to him.
Therefore, behold, I will allure her, and bring her into the wilderness, and speak comfortably unto her…. And I will betroth thee unto me for ever; yea, I will betroth thee unto me in righteousness, and in judgment, and in lovingkindness, and in mercies. (Hosea 2:14, 19)
They find mercy in the wilderness and where it was said of them in the previous chapter that they were no longer his people, they are now called his people and were they were called a people without mercy, they have now obtained mercy.
And I will sow her unto me in the earth; and I will have mercy upon her that had not obtained mercy; and I will say to them which were not my people, Thou art my people; and they shall say, Thou art my God. (Hosea 2:23)
This is exactly what Peter confirms in the NT.
But ye are a chosen generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light: Which in time past were not a people, but are now the people of God: which had not obtained mercy, but now have obtained mercy. (1 Peter 2:9-10
By citing Isaiah 54 to the Galatians Paul is revealing to them that Ephraim has become the husbandmen of the vineyard and by being scattered throughout the world they bring the gentiles to the true God, albeit they lose their identity in the process and become the lost tribes.
So how can Israel be gathered because they are redeemed and yet sown among the nations?
I would argue this points towards the great commission of those who have been gathered to Christ.
Peter, the apostle to the circumcised, along with his brethren, took the gospel to the lands where they knew the ten tribes dwelt; Citing Hosea affirms this and well as the historical accounts, such and Flavius Josephus. These descendants of Ephraim had become a vast people, which stems from the promises of fecundity in God’s covenant.
And when Joseph saw that his father laid his right hand upon the head of Ephraim, it displeased him: and he held up his father's hand, to remove it from Ephraim's head unto Manasseh's head. Joseph said unto his father, Not so, my father: for this is the firstborn; put thy right hand upon his head. And his father refused, and said, I know it, my son, I know it: he also shall become a people, and he also shall be great: but truly his younger brother shall be greater than he, and his seed shall become a multitude of nations. (Genesis 48:17-19)
I will hiss for them, and gather them; for I have redeemed them: and they shall increase as they have increased. And I will sow them among the people: and they shall remember me in far countries; and they shall live with their children, and turn again. (Zechariah 10:8-9)
But the clincher abides in the prophecy to the sons of Joseph in the last days. Remember, the sons of Joseph are Ephraim and Manasseh.
And Jacob called unto his sons, and said, Gather yourselves together, that I may tell you that which shall befall you in the last days…. Joseph is a fruitful bough, even a fruitful bough by a well; whose branches run over the wall: archers have sorely grieved him, and shot at him, and hated him: his bow abode in strength, and the arms of his hands were made strong by the hands of the mighty God of Jacob; (from thence is the shepherd, the stone of Israel) (Genesis 49:1, 22-24)
The shepherd and the stone of Israel are Christ. Jacob was looking down to this age, the age where the leadership of Israel was taken Judah and given to the sons of Joseph, Ephraim taking the privileges of the firstborn.
Behold, I will bring them from the north country, and gather them from the coasts of the earth, and with them the blind and the lame, the woman with child and her that travaileth with child together: a great company shall return thither. They shall come with weeping, and with supplications will I lead them: I will cause them to walk by the rivers of waters in a straight way, wherein they shall not stumble: for I am a father to Israel, and Ephraim is my firstborn. (Jeremiah 31:8-9)
The next gathering in scripture is not to the land of Israel but to Father in heaven, the true promise land.
Citing Hebrews 11 affirms a heavenly county, not heaven itself.
When the Son of man shall come in his glory, and all the holy angels with him, then shall he sit upon the throne of his glory: before him shall be gathered all nations: and he shall separate them one from another, as a shepherd divideth his sheep from the goats. (Matthew 25:31-32)
Christ returns to earth, not to heaven.
Behold, the days come, saith the LORD, that I will raise unto David a righteous Branch, and a King shall reign and prosper, and shall execute judgment and justice in the earth. In his days Judah shall be saved, and Israel shall dwell safely: and this is his name whereby he shall be called, THE LORD OUR RIGHTEOUSNESS. (Jeremiah 23:5-6)