Is there a way to get a CSV or TXT etc file of my games?

With nearly 1,500 all told [600+ on GOG], it's difficult to remember or check if I already have a title—many bought in bulk in bundles or sales, so not lodged in the old memory.

You can get a list by using third party tools developed by the community:
Gogrepo
Adalia fundamentals

What I currently use (as I have thousands) is collectorz.com's game software. It's ok, costs a bit but has search for cover and metadata. But I will be moving to my own database at some point. I would really not recommend using excel, at some point you will find it unwieldy and possibly corrupting your data (mine I must have sorted but only a few columns). Databases are more effort but better in every way - my visual db is a good free one.

Ultimately you won't get this kind of thing from the stores as it's in their interest to get you to buy things, not to reduce the catalog.

I recommend MaGog
For more information check mrkgnao's thread

Thanks for your suggestions Nightcraw1er, much appreciated—I'll check them out shortly. Making my own DB is actually why I posted. I currently have a lot of them in Excel, but it's not ideal for slicing and dicing or holding a lot of info about a game—eg my notes about patches or mods or handling quirks, or just notes or quick review summary for myself.

I already use Access DB, so I'm good for resources. Altho my visual db does look very friendly, something I can't accuse Access of! Well it has the opposite effect on me. If I can't establish quickly whether or not I have a game, I'll pass on their latest offer unless I have time to burn—and that's rare.

Thanks again! Thanks Mike, I'll check those.

Magog have gone to legacy mode.
Newer games are not recorded by Magog database now.

I suggest gogrepo.py.

I wasn't aware..
Thanks for letting me know!

I use a similar solution.
I have games in GOG, Steam, Humble Bundle, itch.io, FireFlower Games, Origin, Ubisoft, and some minor websites.

I record order history (including freebies) for each store, and backup everything I can download.
Before I buy any new games, I use a simple script to check if I already have that game.