Buying something from a company that gets it products made by a third party that uses child or slave labor would not be supporting child or slave labor, just the company who paid to have the products made?
There is more direct responsibility here. A company decides where to buy their products from.
The vast majority citizens in a nation, and I include all democracies that I know the specifics of here (we get to vote on a limited selection of candidates every couple of years based on at best incomplete knowledge of what we know has already been done), don't really have a say in the specifics of what their government is doing.
As to your comment about "native" people, would you point to me on a globe which country was not formed through force of arms in all of human history...take your time....I'll wait.
There is a fine line between doing what is necessary to unify warring groups into a stablelish union and unnecessary violence or oppression. In our cases with the natives, the vast majority of people who are well informed of the specifics would agree that the line has been well crossed. We're talking about things that have been done within the last century.
Anyways, this is side-tracking the point.
People are eager to talk about 'China' in broad strokes, but I'd rather they focus more on the current Chinese government.
Because GOG is not selling games to the Chinese government, they selling games to Chinese citizens who should be given access to purchase them. Really, given that the vast majority of those games are not from China, you should be glad that they are taking such an active interest in parts of various cultures beyond their borders.