GOG sells a lot of game who were made available by those who now hold the rights to these games, not those who developped them. Basically you got people legally making money off the work of others, while those won't get anything more than what they got as part of their salary.
That is a weak argument in regards whether wanting to pay or not. Although I like to see original contributors get the cash for all sales, it is pretty hard at best to get a contract that stipulates that correctly.
First of all, many games are basically paid for by bigger publishing houses and they contract that out to developers. Granted, maybe not as prevalent for the times most of these gog games were made, but there's bound to be some of those deals around that time. Sure, the devs made the game, but they wouldnt have had the chance to make it without the publishers money to fund them.
Disregarding publishers and just focusing on the game developers. Who should really own the right and get money for sales? The studio, the team that worked on it, just the game designers? What makes it more right ( or wrong ) that the studio keeps royalties whilst those who worked on it may now have left elsewhere. What makes it more right ( or wrong ) that an individual can just leave a company and claim all royalties with him when it was the studio that gave him the opportunity to make such a game?
What if the game studio/developer is out of business?
I guess it depends where your support falls the most. Im quite happy to see when game studios do get the fame and money that they get to keep most of the profits, but again its not such a black and white picture in the real world and using that as an excuse to not pay for anything is poor. I'd much rather understand ( although not necessarily agree all the time ) an excuse like "they're overcharging such an old game by insane amounts" or "no one actually sells this game I want".
Im sure that those who want services like gog to flourish will support it and those who dont will most likely keep digging elsewhere to get their hands on it :)