First off let me state that I am totally amazed at what GoG has been able to do! I just was invited to the Beta, and I was shocked at how well it integrates all of my different clients. I just can't say enough about how awesome CD Projekt Red is for taking on this task. That being said...

After the initial elation over a complete gaming library on one screen I started to notice something. My Xbox library is far from complete. I have seen other similar posts, but the behavior does not match those. There is not one reason for the titles that are missing on my GoG library. Though I think one thing I have noticed is that Xbox 360 Games are not there. There are other titles as well that are missing, but are not part of Game Pass (I don't have game pass). There just does not seem to be rhyme or reason for why a title is not listed. What is amazing is that games I purchased the disk for (not many) are shown. Anyone else seeing this problem?

Please understand that I still think this is a massive achievement by CD Project Red and GoG games. I just want to see if I was doing anything wrong, and is there a way I can make this better.

For games to appear in the Xbox integration, you must have played them at least once using the linked account, no matter if they're physical, digital or from the pass.
This is because while they may appear as "owned" in the Microsoft account, it doesn't appear in your Xbox profile unless you've launched them at least once.

This should be in a FAQ or something like that: I was going to report a similar issue.

This is something I've read elsewhere too... however, it *still* can't be the complete picture. I've been firing up games in order to make them register in Galaxy, but while some appear straight after, others have only appeared on a later date, and other still haven't appeared at all days later.

Is it Microsoft's end, or GoG's end that's causing this?

For example, of the 9 Call of Duty games I own and have listed in my games (mainly on disc, across 360 and XO), only CoD:MW3 refuses to show, the rest are fine.
[EDIT: all 9 CoD games show, MW3 was under 'M'!]

Of the 5 Castlevania games: 3 appear, 2 don't (including Harmony of Despair which I played for half an hour to try and encourage it to appear lol)

My understanding is Microsoft's API is... fucked, for lack of a better word.

Certainly not.. Every other websites and app is running properly.. This app is slowly been abandoned because of another overhyped game called Cyberpunk