hashishen: Dear rmaertin, I think you mean freeware. Abandonware is an abstract concept and GOG cannot put games that are left without support by the developer on the site, as it would be against copyright laws.
rmaertin: I basically was referring to games as available at "Home of the Underdogs", for example. Whatever is available for free on that site could be made available for free on GOG. To cover the expenses of wrapping it into an installer/emulator and gathering additional materials, GOG could ask for donations.
But I agree that it might not be as simple as that :)
Abandonware does not legally exist. The rights to many of the franchises that are available at HotU have since been bought out by larger companies, most notably EA. They won't bother going after those distributing and downloading it for free because the potential payout would not be worth the legal fees and chance of losing. However, if they are being sold, the company would have a much better case and prospects for profit and so they may well press charges.