So by my count, GOG is one game away from the big 300. Does the site have anything interesting planned?

do you think they know that? :P :D

When they reached the 100-game mark, they released a freeware game, Teenagent. There was nothing special for the 200th game released, though, and I don't think there will be for the 300th, either - after all, these aren't as big landmarks as the one hundredth game.
Maybe for the 500th, but not sure.

I'd prefer they save the effort for GOG birthday in September ;)

I don't think so, but as the 100th game, they added Teenagent to the catalogue (as a new free game), but there wasn't any surprise release as the 200th game.
The 99th game at GOG was Septerra Core, and at the end of the news post, it said: "And if you haven't noticed this is the 99th release on GOG.com :)" - so they hinted there would be some surprise. There wasn't any clue at the 199th game, nor this time.

Are you suggesting that we question their omniscience?