Gulls are not as gray and flap their wings more slowly and smoothly and lack the tube on top of the bill. The gray morph of the Northern Fulmar is stockier and has a yellow bill. Flesh-footed Shearwater has pink legs and bill. Short-tailed Shearwater is very similar but is somewhat smaller with a steeper forehead.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.