Faith vs. Law (I)

I still don't understand why Paul quoted Hebrew Scriptures, of course in Greek, to the Gentile readers. Did these texts make any sense to the Gentile readers? Would they know these texts, and their subtlety?
I have been reading Galatians and it was Hebrew text free for the first two chapters. (As for the first epistle to the Corinthians, Paul quoted the Hebrew Scripture nearly once a chapter, beginning with the first letter.) When I reach the first quotation, I know this epistle must have been written by St. Paul.
Καθὼς Ἀβραὰμ ἐπίστευσεν τῷ θεῷ, καὶ ἐλογίσθη αὐτῷ εἰς δικαιοσύνην· (Galatians 3:6)
καὶ ἐπίστευσεν Αβραμ τῷ θεῷ, καὶ ἐλογίσθη αὐτῷ εἰς δικαιοσύνην. (Genesis 15:6)

ἐνευλογηθήσονται ἐν σοὶ πάντα τὰ ἔθνη· (Galatians 3:8b)
ἐνευλογηθήσονται ἐν σοὶ πᾶσαι αἱ φυλαὶ τῆς γῆς. (Genesis 12:3c)

ἐπικατάρατος πᾶς ὃς οὐκ ἐμμένει πᾶσιν τοῖς γεγραμμένοις ἐν τῷ βιβλίῳ τοῦ νόμου τοῦ ποιῆσαι αὐτά. (Galatians 3:10)
Ἐπικατάρατος πᾶς ἄνθρωπος, ὃς οὐκ ἐμμενεῖ ἐν πᾶσιν τοῖς λόγοις τοῦ νόμου τούτου τοῦ ποιῆσαι αὐτούς, (Deuteronomy 27:26)

ὁ δίκαιος ἐκ πίστεως ζήσεται· (Galatians 3:11)
ὁ δὲ δίκαιος ἐκ πίστεως ζήσεται. (Romans 1:17)
ὁ δὲ δίκαιος ἐκ πίστεώς μου ζήσεται. (Habakkuk 2:4)

ὁ ποιήσας αὐτὰ ζήσεται ἐν αὐτοῖς. (Galatians 3:12)
ἃ ποιήσας ἄνθρωπος ζήσεται ἐν αὐτοῖς. (Leviticus 18:5b)

ἐπικατάρατος πᾶς ὁ κρεμάμενος ἐπὶ ξύλου, (Galatians 3:13c)
πᾶς κρεμάμενος ἐπὶ ξύλου (Deuteronomy 21:23c)

καὶ τῷ σπέρματί σου (Galatians 3:16c)
Τῷ σπέρματί σου (Genesis 12:7a)
Quoting the texts in a breath-taking speed, Paul was eager to prove that faith is more important than laws. He was so eager to defend his position that his Pharisaic training was unwittingly revealed. Read verse 16. He was defending with a Grammar item.
Now the promises were made to Abraham and to his offspring. It does not say, "And to offsprings," referring to many; but, referring to one, "And to your offspring," which is Christ (Galatians 3:16)

My dear Advocate, I undertand that understanding a person is an extremely difficult task. We pray that with Your gifts of wisdom, counsel, knowledge, prudence and integrity, we may learn from Paul and continue to work for your Kingdom of God. Amen.