Gf and I talking about marriage?

You might just as well get used to "giving in".

You're lying (at least to yourself).

If you're not that religious, and she is, you're making an issue out of something you claim isn't that big of an issue for you.  So here's what you say to her:

Darling, you know I love you, and therefore since this religious thing is much more important to you than me, I'll indulge you.  We'll do whatever you want when it comes to religion.  I'm sure there will come times when something is very important to me, but not that big of an issue with you where you'll indulge me.  Am I right?

Let GOD decide! Mark S.O. on one side of a coin, and C.C. on the other. Place it in a clear jar, fasten the lid. You and she BOTH hold and shake the jar, while praying to t\GOD to use His INFINITE POWER to nudge the coin to show HIS coice.

Then stop shaking the jar, and  gaze with reverent awe on the answer GOD has revealed.

IF you and she don't believe that GOD has the Power to nudge a coin, or that GOPD actually CARES which religion a child is raised in, then you are atheist enough not to care, either.

I have "Nelson's Guide to Denominations" book that generally describes all of the Christian religions in the USA, that is 2000.  Neither religions with the first words you  have used are in it.

I would go more for hers. The Catholic religion that was not dreamed up until 304/5 AD by 29 very lazy men that did not want to keep obeying all that is required of us to be Christian.   The Catholic religion has never even taught the real Second Commandment that requires us to pray only to GOD!  That If we pray to anyone one or anything other than GOD; it would not be answered. And Catholics keep insisting as me that "GOD dose answer prayers anymore."!  GOD dose answer mine.

The Catholic religion has split the tenth commandment into two commandments, then claim they have ten.

Are you ready to argue over this as Catholics do? Only wanting to do as little as they think they can get away with?   Would you be willing to start obeying the real Second Commandment, and pray only to GOD...? 

This is why the Catholic religion has always been listed apart from all the other religions. Because it is so much more disobeying the Bible than others.

Read 1 Corinthians chapters 5 & 6. The Catholic religion and all others that are permitting homosexuals into them; have left GOD and are not Christian at all. They are now devil following religions.

So her religion is far more obedient to the Bible.  Your children would be safer and more honest with hers. 

What is more important to you, your pride, or your children and family's safety? 

Pray on it instead of getting advice from people. God's truth is complete.

I am sorry to hear about your dilemma. You know, these things ultimately don't matter. What matters is your relationship with Christ and knowing him. Seek him out first with all of your heart, soul, and mind and you'll see how trivial these things are. Jesus himself was asked about certain holidays and festivals, but he was more concerned with more important things. Also, the people who stood to accuse him of not following the sabbath didn't understand that he was there as the fulfillment of the sabbath.

In the same way, you are holding some random date that people as a whole have come together to say was when Jesus was born. We don't know that. There is nothing in history that shows that as a fact, and ultimately it doesn't matter. What does matter is the family time and sharing of love and grace towards each other. It sounds like you need to have a bit more grace toward each other if you want to go forward in this relationship, but I would suggest you first do what I suggested above and really lean into understanding who God is.

All your life you probably grew up in that church hearing people tell you who God is, but he is one way, and you should have a first-hand account of him without influence by anyone who would mislead you. There are some huge doctrinal differences between what the bible says and what some of those churches believe/do. You'll note I'm not asking you to join a specific church. I'm just saying to do your research. Jesus had a prophecy concerning the future generations (aka us), and he said that they would follow many false teachers and prophets who "tickle their ears" with their words. Instead, seek God first, as that is what he asks of you.

Obviously, she wants Eastern Orthodox. Serbian, if possible. There's a difference in Holy Days because different calendars are used. If this marriage is going to work, someone has to change.