As the title. Most GOG DLC packs specifically state on the store page that the game is required; this one doesn't. So is the Season Pass the game + DLC, or just all DLC?

No, it doesn't include the game. It's a discount on the expected release price of the three DLCs, which will be about $20 each, as Floashpoint was.

The reason the season pass doesn't say that it requires the base game is probably because the season pass isn't actually a game. It's instead a voucher for the game content you'll be getting as a result of buying it. So when you see that the season pass includes Flashpoint, you can see on the Flashpoint page that the Flashpoint DLC does require the base game.

So, no. Base game not included.

For the full pack of Game + Season pass, you need to select the Mercenary Collection under version/edition on the game's store page.