I'd like to play Syndicate under Windows 98/DOS. Can you just copy the files and play it on old hardware? And is everything working correctly, for example some games used audio-streaming from CD, which wouldn't work with just copying files.

The original Syndicate is for DOS, and the GOG version just runs on DOSBox. It is not just "copying the files", but if you are trying to run a vintage PC, I'm guessing you know your stuff and only mean if the files can be obtained from the GOG install and used without modification... So, yes, you should be able to run it in a '90s computer.

And it runs completely from disk, there is no CD image.