Here's a nice easy question for you guys who want to get your rep up...
How do I know downloading these games is legal and I'm not going to get done for piracy? That's basically my only question.

Because GOG is selling them with permission from the publisher and developer, and those developers and publishers actually make money from the deals.

wow... that was a quick response.
ok thanks, I might be buying some games then...

Do some people seriously think GOG might be some kind of piracy front? GOG is owned and operated by one of the best publishing and development firms in Poland, CD Projekt (the people responsible for the Witcher games). You cannot get much more "above board" than GOG is.

ok that's cleared up any doubt : )

GOG is a legitimate company that has been in business for approximately 2 years. You can read in many blogs, and online gaming resources about their business and their view of DRM free gaming if that helps.
Here are few links to read.
Link 1
Link 2
Link 3
Hope this help assuage your concerns.

What likely happens is that people once more act on things they don't understand.
Certain piracy/abandonware sites complain that GoG is selling games that they are providing for free.
Certain piracy/abandonware sites point out that there is no way GoG can get the original creators' permission for a lot of games (and ignore things like the rights to things being sold...)
People point out that GoG kind of 'jacks cracks and the like.
So when they hear stuff like that, they don't think about what they are saying and just make kneejerk comments and decide to immediately make all-or-nothing decisions.

Welcome. Don't worry, everything is legal here, except for the things which are not. :p
One question for you, mate. How do you get done for piracy?

Speaking only for myself, I like to get done for smuggling and the occasional kidnapping.

I guess sometimes people jump to conclusions. I mean, if you see a GOG banner or ad in a site of dubious reputation and then you read GOG is from Poland (not that I have anything against Poland, but people make poorly based assumpitions of countries they don't know much about) and then you start wondering how legit the site is. We have nothing else to base or judgement on when we look at a site, so we do searches and ask questions until our fears are dispelled.
When I bought my first game (Original War) my antivirus acted up. I then read about the problem on the forums (hell, I think it's the issue that brought me to the forums) and I couldn't help wondering if GOG was installing stuff on my computer or collecting credit card data or if it was a front for any of those online scams you read about nowadays, using old games instead of wallpapers or useless software to get to my pockets.
Luckily, I started seeing a thriving and friendly community, a company that cared for it's customers, zero strange going-ons with my credit card, a brilliant idea that it was odd no one had thought about before and a love for old games that was downright infectious.
Been here ever since.

To be fair, at first glance this place does seem a bit too good to be true and there's no immediately visible indicator to indicate a connection to CDP

Abandonware sites are a gray zone, GOG is a completely "green" zone as of piracy. So purchases here are completely legit. It's that simple, really.

Since you noted the idea of moving someone rep up, you ought to give the person of your choice a rep bump for helping to answer your question! :o)

Quite simple. A lethal suspension of a person with the application of a ligature. At least that's what they used to do. Don't know any new methods implemented in the British navy.

Probably the easiest way to be absolutely certain that GOG are legit is the fact that we don't yet have EA or Lucasarts games here. I hardly think they'd overlook two of the most demanded publisher's titles if they were trying to make an illegal fast buck.
Oh and don't forget to mark the post you think helped the most as the solution.

No they're not. It's just that it's unlikely that most publishers are going to focus their anti-piracy efforts on titles they're no longer trying to profit from.