Paul here is speaking of pagan Greek gods.
From the NABRE:
Paul’s Speech at the Areopagus. 22 Then Paul stood up at the Areopagus and said:[a]
“You Athenians, I see that in every respect you are very religious. 23 For as I walked around looking carefully at your shrines, I even discovered an altar inscribed, ‘To an Unknown God.’** What therefore you unknowingly worship, I proclaim to you. 24 The God who made the world and all that is in it, the Lord of heaven and earth, does not dwell in sanctuaries made by human hands,**
From the NABRE’s footnotes:
In Paul’s appearance at the Areopagus he preaches his climactic speech to Gentiles in the cultural center of the ancient world. The speech is more theological than christological. Paul’s discourse appeals to the Greek world’s belief in divinity as responsible for the origin and existence of the universe. It contests the common belief in a multiplicity of gods supposedly exerting their powers through their images. It acknowledges that the attempt to find God is a constant human endeavor. It declares, further, that God is the judge of the human race, that the time of the judgment has been determined, and that it will be executed through a man whom God raised from the dead. The speech reflects sympathy with pagan religiosity, handles the subject of idol worship gently, and appeals for a new examination of divinity, not from the standpoint of creation but from the standpoint of judgment.
‘To an Unknown God’: ancient authors such as Pausanias, Philostratus, and Tertullian speak of Athenian altars with no specific dedication as altars of “unknown gods” or “nameless altars.”