I'm more than halfway through Gunpowder now (248/205 to go), no gems or $$$ spent. While the requirement is shocking at first, given the length of the event it's perfectly achievable.
I'm hopeful that Krait and Millenial will be easier - given that they're not new (though I don't have them yet) that seems pretty plausible. I can't see getting Pyrotechnic without spending, since I'm sure it'll cost even more than Gunpowder, but 6 new dragons for free is pretty nice.
Combined with the Amusement Park, that could be 8 if @_TheSpaceBeing_ is right and Terrifying is in reach with ads - did those calculations include getting Headstrong/Grapple, or just going far enough for the keys? Personally I hope to get Grapple/Terrifying, but I already have Headstrong so will just go far enough to get the Grapple key with him.
Could even be 10 if I complete the collection to get Strongman and am lucky enough to land the breed for Wastelander. Though those are more likely to need some gems to speed up hatching/breeding.
All that, and I am now in the process of empowering a Heroic for the very first time! (THAT cost ~$50 in coins/gems to blitz the Weather Machine, buy Winter Sun/Moon orbs, and speed up empowering enough to get Hyperion/Uller done by race end.) Good start to the New Year as far as I'm concerned, especially since all three of the other race prizes were new to me too, and the previous Alliance Chest finally got me enough orbs for Vulcan...