Same. I have several old spinners full of CDs and DVDs but converted over to the thumb drives. Basically have my entire gog collection on a handful of 'em.

How are them discs holding up? Are they factory printed or home burnt? How old are they? Do they still work?

Most of them are just store bought blanks that I burned with my (ancient) external 16x Memorex drive. Some of the RW discs won't load files any more, but most of the CD-R and DVD-R discs work just fine. The ages range from a year or two all the way to over 10 years old.

I don't have money for an external HD. I do own one old 1TB HD (external) and have backups of about 20 of my favourite games on GOG but since I own about 600 then there is no way I could backup all of them.

I should also add here that in their TOS GOG does state that they will give us 60 days of warning before shutting down so there is that. Clearly that's not a guarantee but knowing them I think they would honor that promise.