Hi Podo.
Something to remember (IMO) is that Pauls letters are not 'first contact'. It was generally people who were already Christians who were reading them, people who had already been preached to. Therefore he is not laying out the entire Christian belief system in them, as if for the first time. He is giving reminders and 'patching up' places where leaks had developed in their faith, so to speak. You can't assume that those reading his letters knew nothing and were getting all their knowledge from the letters of Paul or other Apostles. This would include their understanding of Jesus nature, which would have been something covered pretty early on in their 'conversion'. It's no surprise then that the letters speak of Jesus nature as if they are addressing those who already know, rather than those hearing for the first time. The Deity of Christ is certainly there, but it's not heralded like a new revelation with the stupendous exaggerated obviousness that some today insist is required (despite their not requiring for many other matters of doctrine which they are happy to deduce as solid based on several Scriptures weighed together).
A Jew awaiting the Messiah had already been in for an incredible shock, in that Jesus was nothing like the Messiah they were expecting nor did he do the things they expected the Messiah to do. So appealing to what Jews apparently expected isn't really valid, as most of what they expected and most of their understanding was wrong anyway. Any Jews who converted to Christianity would have had to have radically altered their expectations. Paul himself talked about this. That's why most Jews rejected Jesus, because he wasn't what they expected.
Jews had more than just Psalms 2 to go on. They had all the text regarding the Messiah to consider, including those which suggest he would be God. Few Jews understood this, but so what? What does that prove? Paul said that they were largely insensible regarding the Messiah, so their expectations - mostly false - are irrelevant, and can not be used to dismiss the notion of Jesus being God.
So, in summary, this hypothetical Galation Jew of yours would not be hearing about Jesus and his nature for the first time from such a letter and his supposed expectations of the Messiah are irrelevant, firstly, because they would almost certainly have been wrong and, secondly, he would have had to abandon them some time previously, when he was first preached to.
