Based on this announcement I would hope it supports the Series 1; I don't really want to upgrade to a 2:
http://pokemongolive.com/en/post/headsup/
"Additionally, the Pokémon GO app for Apple Watch encourages workouts with summaries of your activity, which fits in nicely both with Niantic’s mission to get you moving as well as with the way many people already use their Apple Watch."
There's no mention of Series 2, GPS, or anything that makes it seem like the new model would be required. Guess we'll find out when the app comes out...