Galatians verse by verse

2:3 Yet not even Titus, who was with me, was compelled to be circumcised, even though he was a Greek.
Note:
Indicating that the leaders at the church in Jerusalem did not consider circumcision a criteria for gentiles to follow Christ.

2:4 This matter arose because some false brothers had infiltrated our ranks to spy on the freedom we have in Christ Jesus and to make us slaves.
Note:
Circumcision became an issue because of these false brothers.
Freedom in Christ in the context following 2:3 is about circumcision, clearly not about the 10 commandments or any other parts of the Law of God given through Moses which, at this point in the letter, were not mentioned.
Slaves: Same here. It's about enslavement by circumcision.

2:5 We did not give in to them for a moment, so that the truth of the gospel might remain with you.
Note:
The truth of the gospel matters most.
A few verses about the gospel (the good news):
Was foretold in Isaiah 52:7-62:12
Yeshua identified Himself as the savior by quoting Isaiah 61:1 in Luke 4:16-21; Isaiah 55:1-7 in John 4:1-26 and John 7:34.
Yeshua fulfilled Isaiah 53:1-12 by dying on the cross.
Paul echoed Isaiah 52:7 in Romans 10:15; Isaiah 52:11 in 2 Corinthians 6:17; Isaiah 52:13 and Isaiah 53 in Philippians 2:6-11; Isaiah 54:1 in Galatians 4:27.

Isaiah 52:1 mentioned that the uncircumcised and defiled will not enter again. Yet in Isaiah 56, the observance of Sabbath and keeping one's hand from doing any evil rather than circumcision is the prerequisite for admission for gentiles and, in Isaiah 57-59, returning to God from iniquities and sins and observance of Sabbath for Jews.

I was so stubborn to have insisted on studying verse by verse and thus neglected to address the controversial verses which are the heart of the problem with Galatians. So I'm going to jump to those.

Galatians 4:21-31
4:23 ...but his son by the free woman was born as the result of a promise.
The promise was made to Abraham: Genesis 15:13-14...Know for certain that your descendants will be strangers in a country not their own, and they will be enslaved and mistreated four hundred years. But I will punish the nation they serve as slaves, and afterward they will come out with great possessions.
Then reconfirmed with Isaac: Genesis 26:3-5....for to you and your descendants I will give all these lands and will confirm the oath I swore to your father Abraham. I will make your descendants as numerous as the stars in the sky and will give them all these lands, and through your offspring all nations on earth will be blessed, because Abraham obeyed me and kept my requirements, my commands, my decrees and my laws.
Then reconfirmed again with Jacob: Genesis 28:13-15 I am the Lord, the God of your father Abraham and the God of Isaac. I will give you and your descendants the land on which you are lying. Your descendants will be like the dust of the earth, and you will spread out to the west and to the east, to the north and to the south. All peoples on earth will be blessed through you and your offspring. I am with you and will watch over you wherever you go, and I will bring you back to this land. I will not leave you until I have done what I have promised you.
And again with the Israelites after fulfilling in part what He promised Abraham: Exodus 19:4-6 You yourselves have seen what I did to Egypt, and how I carried you on eagles' wings and brought you to myself. Now if you obey me fully and keep my covenant, then out of all nations you will be my treasured possession. Although the whole earth is mine, you will be for me a kingdom of priests and a holy nation...
Note the similarity of the terms made with Abrahams: Genesis 17:1-2 ...I am God Almighty; walk before me and be blameless. I will confirm my covenant between me and you and will greatly increase your numbers.

One would notice there is a catch to His promise i.e. to obey Him fully i.e. walk before Him and be blameless.

They were to obey His words i.e. what we called the ten commandments in Exodus 20:1-17. And that's Sarah's son i.e. obedience to the promise.

And various laws were given, such as temple worship and animal sacrifices, such as laws to elaborate and to protect the people from offending the ten commandments, such as laws to separate the people from surrounding nations. To observe these laws, without understanding of the ten commandments and the promise that comes with, is like Hagar's son i.e. mere works with no understanding of the promise.

Galatians 3:21 Is the law, therefore, opposed to the promises of God? Absolutely not! For if a law had been given that could impart life, then righteousness would certainly come by the law.

To continue if God willing.

Hagar: a Egyptian slave girl (Genesis 16:1). Interesting enough Egypt was where the Israelites came out from about 400 years later.
Hagar: she came on the scene not by her own but by none other than the faithlessness of Abraham and Sarah (Genesis 16:1-2). One would wonder why because Abraham is the father of faith after all - this is to explain separately.

Sarah could not have children: a metaphor of man who could not stop sinning (Galatians 5:19-21) i.e. to bear fruit of the Spirit (Galatians 5:22-23).
Sarah relied on Hagar to bear a child: a metaphor Paul used of man to rely on the law (Galatians 3-4).

God gave His promise to Abraham and Sarah that they would bear a son (Genesis 17:15-19): get the metaphor?