The current rights owner of the games sold on GoG.
I doubt that original creators/developper get's any royalties from the games they create/program, no matter how old or recent they are anyway. Maybe some big names like Molyneux or Will Wright but I doubt many other.
I really hope not, after the X-Com FPS and the Ultima "pseudo-RTS" (and let's not talk about poor Larry Laffer horible fate) I think I have had enough old licence desecration for a life time.

In general that's true, but let's not forget that sometimes the rights holders are the original creators. A quick example of this is the Tex Murphy-games.

Or The Longest Journey by Funcom. Still waiting for that final part!

New game or old game, usually creators get nothing from sales. Thats the way it is in the big entertainment business. Game studios are payed in advance by publishers for development of the game and publisher then keeps all the profit.
As for GOG specifically, I'd say we, the gamers, are benefiting from the sales. It gives publishers reason to still sell old titles instead of forgetting them and making new crappy spinoffs. Many of the older classics are really hard to find, or was before GOG came around.

It's a complex process, but the gist of it is something like this:
- GOG receives the funding electronically from the end user through their chosen payment gateway.
- GOG then automatically divides the funds between themselves and the IP squatter in accordance with their agreement.
- At the end of every month, GOG then sends the total sum of money accrued on behalf to the IP squatter as used bank notes in a briefcase via courier.
- The courier meets with one of the minions sent by the IP squatter and identical briefcases are exchanged under the table.
- Once the minion has returned with the loaded briefcase, the IP squatter carefully invests the money in the company's C&H division.
This process isn't entirely necessary as it's an entirely legitimate transaction. But it's all a part of the service that GOG provide to help the IP squatter retain their sense of illegitimacy.

As stated, most of the original developers aren't going to reap any benefit from the sales on GoG.