How do Jehovah's Witnesses understand Galatians 1:11-12?

Jehovah's Witnesses believe that Jesus was a man during his time on earth, but not just any ordinary man. He was the Christ, he was God's Son, and he was perfect. What Jesus taught was not of his own originality but was from God, his Father. (John 8:28; John 14:10)

Before Jesus came to earth and after he was resurrected, he was a mighty spirit creature. The Apostle Paul received his gospel after Jesus was resurrected, so Jesus was no longer a man then.

1 Peter 3:18 (NWT)
For Christ died once for all time for sins, a righteous person for unrighteous ones, in order to lead you to God. He was put to death in the flesh but made alive in the spirit.

Acts 13:34 (NWT)
And the fact that He resurrected him from the dead never again to return to corruption, He has stated in this way: ‘I will give you the expressions of loyal love promised to David, which are faithful.’

1 Corinthians 15:45 (NWT)
So it is written: “The first man Adam became a living person.” The last Adam became a life-giving spirit.

2 Corinthians 5:16 (NWT)
So from now on we know no man from a fleshly viewpoint. Even if we once knew Christ according to the flesh, we certainly no longer know him in that way.

Sources:
jw.org > Bible Questions Answered > After Jesus’ Resurrection, Was His Body Flesh or Spirit?
What Can the Bible Teach Us? > Chapter 4: "Who Is Jesus Christ?"

To find the Jehovah's Witness perspective, I looked at their “Insight On The Scriptures” book, vol. 1. The first indication is that they have no subject heading, “Gospel”. They seem to avoid that word, replacing it with “the good news” instead. Their New World Translation of those verses you ask about read,

“For I put you on notice, brothers, that the good news which was declared by me as good news is not something human; for neither did I receive it from man, nor was I taught [it], except through revelation by Jesus Christ.”

So, under the heading “Good News” I looked for anything about Jesus no longer being a man after his resurrection and found nothing, as they do not link those verses with your points at all. A JW answer has already stated,

“after he was resurrected, he was a mighty spirit creature. The Apostle Paul received his gospel after Jesus was resurrected, so Jesus was no longer a man then.”

That was followed with four texts from their NWT to support that claim, one having a mistranslation of the Greek, but that is not the point of this question, nor of my answer. The JW answer agrees with my point that JWs do not see in Galatians 1:12-13 any problem with their doctrine of the resurrected Christ no longer being human.

Next, I looked at the subject heading “Resurrection”, which takes nine pages, but did not find Galatians 1:12-13 mentioned, which is not surprising as it has no bearing on the subject in general or on whether Christ was raised with a glorified physical body, or an immaterial spirit body. There is, however a paragraph claiming that at Jesus’ resurrection he

“was granted immortality and incorruption, which no creature in the flesh can have…” Then, when dealing with the resurrection of Christ’s brothers, it claims that “They must die a death like Christ’s – maintaining integrity and giving up human life forever… Christ’s faithful brothers, who join him in the heavens, give up human life. The apostle Paul shows that they have to have new bodies…”

I quote these bits because they underline the JW doctrine of those going to heaven having to “give up” their physical bodies and that new bodies are created for them for “the soul, the person that is resurrected” to then inhabit in heaven. That is their teaching, which they apply equally to Jesus Christ as to those who are to be with him in heaven.

It would appear that you have raised some points about Galatians 1:12-13 that they have never considered before, but from what they do believe, they would just take those verses as harmonising with their doctrine of physical bodies having to be “given up” before a new spirit body would be created for those going to live in heaven. They insist that a new spirit body had to be made for the resurrected Jesus. The body he died in, and which was laid in the tomb, was never seen again by anyone, including Jesus, who found himself in a new, spirit body, that he could (at will) change to appear to be fleshly, though it was not. Page 786 of the JW ‘Insight’ book, vol. 1, makes that clear. Therefore, you are coming at those verses from a totally different angle to how the JWs view them, and your points will either be misunderstood by them, or dismissed by them as irrelevant, for your approach is incompatible with theirs and vice versa.