In this particular context Paul follows the existing Jewish tradition that claims that the Law was given to Moses by angels (Gal.3:19; cf. also Acts 7:53), in difference from other tradition, that claims that it was given directly by God. That is the meaning of saying in Gal.3:20 that the "mediator" (i.e. Moses) is not mediator of one [God], but of several [angels].
As to the question "one what" in the phrase "but God is one": it is a phrase denoting the same as in Jesus' quote of the Deuteronomy 6:4 ("Hear Israel, God your Lord is one" /Mark 12:29/); it can have two meanings:
1) "God is one [person]"; in which case it can imply God the Father, who is one person, as in Paul's other expression in 1 Cor.8:6 ("for us there is one God, the Father, from whom are all things, and we for Him; and one Lord, Jesus Christ, through whom are all things, and we through Him”). If this is the case (which I think is quite plausible), given the last quote from the 1 Cor. 8:6, it is not at all denied the indispensable and ontologically necessary role of Jesus Christ for the Father to give sonship to humans, which was not available even to Moses, not to even mention to ordinary humans through Moses. Faith in Jesus Christ is ontologically necessary for humans to have access to a renewed divine life as new creations in Christ (Gal. 2:20) and God's adopted sons (Gal. 3:26); moreover, even for God the Father His unique Son and Logos is ontologically necessary for vouchsafing this sonship to humans, no less than uni-co-agency of the Son-Logos is ontologically necessary for the Father to create the universe, for the created universe is simultaneously "from" the Uncreated Father "through" the Uncreated Son (cf. 1 Cor.8:6; Hebrews 1:2), when "from" cannot be separated from "through" (like, to use a famous metaphor, enlightening "from" the sun can happen only "through" the rays of the same sun).
or 2) "one" in this passage can imply just "reality", as in an expression: "the world is one", that is to say, the world is one reality. Now, this one reality of God entails also difference of Father from the Son (and both from the Spirit), and all Three are the one reality of Divinity. And in the context of the passage from Galatians the talk is about the new covenant with God, a new life of faith that is not possible through circumcision and Law, but only through baptism, but the baptism is in one shared name [God] of all Three - the Father, the Son and the Holy Spirit (cf. Matthew 28:19).