At this point, I would guess that almost everyone who will buy KOTC on your site has already done so and your sales have declined heavily since release. Meaning, those who cared enough to search for games like yours, or frequent the places where such games are mentioned, have already come upon it at some point. Thus, the value of GOG is that it shows your game to thousands of more people who have never heard of it, because their interest in games like this are casual. They may have played the gold box games like Dark Sun in the past, and are on GOG to live their nostalgic youth. An untapped market, I would assume, as they are not active in their search for games, but on GOG they will passively find them. Therefore, you would gain sales that you would otherwise probably never get. Certainly, you will lose some revenue from people who will buy it on GOG instead of on your own site, but I would imagine that the amount of "casuals" who get to know that KOTC exists through GOG would more than compensate for the lost sales. I see this as a situation where there's a lot of potential buyers you'll never reach unless you sell your games through something like GOG, because the visibility of your games is extremely low among more casual buyers, but most of the hardcore buyers have already bought what they wanted, so the amount of revenue you might lose from lowered profits per game would be compensated by greater volume of customers.
If your sales haven't declined, and have been more or less the same each month since release, then indeed it'd probably be a bad idea to sell it on GOG or Steam. But if, say, your sales have halved every half a year, then I would strongly suggest trying to make a deal with GOG or Steam(the greenlighting feature might be of use on Steam).
I guess it ultimately depends on if you can afford to take the risk. If you are dependent on the sales you make on your own site, it's a bad idea to risk losing that revenue by gambling on it. But if it's merely a bonus that you don't really need to survive, I think the potential benefits of a large amount of extra sales would be worth the gamble. There's also the fact that the more people buy KOTC, the more people will know of KOTC2 and thus you'd probably get more revenue from it when it is released. With that in mind, perhaps you might want to try GOG or Steam on the same year you intend to release KOTC2, since if it's fresh in their mind, they're more likely to make a purchase, than if it's years in the future.
Worth thinking about, I believe.