The Blue-gray Gnatcatcher is so small but long-tailed that it is only likely to be confused with other gnatcatchers. Male Black-tailed, California, and Black-capped Gnatcatchers have variable amounts of black (depending on season and age) in the crown while Blue-gray Gnatcatchers lack any black in the crown. Female gnatcatchers, other than the Blue-gray, have their upperparts tinged with brown. Black-tailed and California Gnatcatchers have blacker tails when viewed from below and typically have grayer underparts.
Length and wingspan from: Robbins, C.S., Bruun, B., Zim, H.S., (1966). Birds of North America. New York: Western Publishing Company, Inc.