The short answer: Both are gravel bikes.
The Grail is optimised for smoother off-road surfaces with the Grizl better suited to rougher terrain.
The Grail is more efficient on-road but slightly less capable than the Grizl off it while the Grizl is more comfortable on the rough stuff but not as fast on the smooth.
The long answer: Read the
blog post
for a detailled breakdown of the differences between the two bikes.