Is there an official map legend (mountains, forests, desert, etc...) for the hex crawl?

Not currently, as the terrain types are fairly self-explanatory 

What is difference between the Pine Tree Hex and the other Tree Hex?  What kind of terrain is Gallowstone?

Hey sorry for the late reply! There's functionally no difference between the two forest types, just a difference in scenery. As for Gallowstone, it is considered badlands, full of geysers and hoodoos.

Thanks for the reply.