The only DRM we have is a license key, but that's absolutely non-optional because of how we distribute the game and updates to the game. If we made a non-license-key version for GOG, their customers would not be able to receive any future updates from us without it reverting them to a license-key-enabled version, and thus locking them out of the game they'd bought.
If GOG supports license keys, then it's a possibility, but if by "no DRM" they mean "just a raw executable and nothing else," that simply can't ever work with our games. It would only work if we never ever released another update for a specific game again, or if we stopped doing public demos of our games that can be updated to the latest version of the game, etc, etc. I highly doubt that GOG would provide enough revenue for us to justify having an entirely separate update path for them alone; right now Steam is the only vendor that meets that particular criteria for me.
Best of luck to them, though; it's great being able to get old games from them, and if selling newer games helps them in their goals then great.