seppelfred: Ah, come on. External hard drives are not very expensive. If you can buy "thousands of games", you can afford it.
Depends how big you need.
I have total 7 TB reserved for my GOG offline installers (one 5TB drive and another 2TB drive, dividing the installers on them), and right now I am going beyond their storage space, ie. I should buy a third hard drive, or replace the 2TB drive with e.g. a 5TB drive, pushing the storage to 10TB total...
Having the installers divided to two (or more) HDDs complicates things somewhat when using e.g. gogrepoc.py, ie. I have to divide the gogrepo manifest file manually to two (or more) manifest files, and then run gogrepo twice on the two separate hard drives...
Sure it would be nice to keep all the installers on one big-ass HDD, but I don't think there are that big 2.5" USB-powered HDDs, and even if there was, it doesn't feel nice to retire the perfectly working 5TB and 2TB HDDs just because the collection can't fit into them, barely.
I have several older working 2TB 3.5" SATA HDDs in the closet, I guess I should take them into use and divide my GOG collection on them.
I still need to research how to make one big volume out of several (USB) HDDs, where I could always extend that volume simply by adding yet another hard drive to the mix... I know it can be done, both in Windows and Linux, but if you do it without any parity to maximize the storage space, then it is even more vulnerable than dividing the files to several separate hard drives manually, as losing one of the hard drives would make you lose the whole volume, ie. all your GOG game installers. Not just the ones which were on that specific hard drive.
Oh well, what would be life without some obstacles?