In the CSB it appears that they are:
[Act 15:17 CSB] (17) "so the rest of humanity may seek the Lord -- even all the Gentiles who are called by my name -- declares the Lord who makes these things
In the NLT the gentiles are a subset:
[Act 15:17 NLT] (17) so that the rest of humanity might seek the LORD, including the Gentiles--all those I have called to be mine. The LORD has spoken--
[Act 15:17 MGNT] (17) ὅπως ἂν ἐκζητήσωσιν οἱ κατάλοιποι τῶν ἀνθρώπων τὸν κύριον καὶ πάντα τὰ ἔθνη ἐφ’ οὓς ἐπικέκληται τὸ ὄνομά μου ἐπ’ αὐτούς λέγει κύριος ποιῶν ταῦτα
What makes this ambiguous perhaps is καὶ can be use as "and" or "even".
[Amo 9:12 NASB] (12) That they may possess the remnant of Edom And all the nations who are called by My name," Declares the LORD who does this.
Brenton LXX 12 (LXX 1:11) that the remnant of men, and all the Gentiles upon whom my name is called, may earnestly seek , saith the Lord who does all these things.
[Amo 9:12 LXX] (12) ὅπως ἐκζητήσωσιν οἱ κατάλοιποι τῶν ἀνθρώπων καὶ πάντα τὰ ἔθνη ἐφ᾽ οὓς ἐπικέκληται τὸ ὄνομά μου ἐπ᾽ αὐτούς λέγει κύριος ὁ θεὸς ὁ ποιῶν ταῦτα
This is from the Dead Sea scrolls of Amos 9:
12 that they may possess the remnant of Edom, and all the nations who are called by my name,” says Yahweh who does this.