White gunk on my saddle?

It is either a type of mold that loves leather, or it is mildew from horse sweat. Being that you call it "gunk" it is probably mildew from sweat (the mold is more powdery in texture). The saddle soap & conditioner will be fine either way. Make sure you check any other leather equip stored in the same area because if it is mildew or mold it will continue to grow if you don't rid of it all. Waterproof your leather goods so after your next ride you simply rinse it off with the water-hose.