Here's a series of pictures taken of the dark-morph Gyfalcon that took
up residence in South Boston for an entire winter some years ago that
shows the general ID marks very well.
Note the chesty, almost round shape with the disproportionately small
head and tail extension when the bird is relaxed, the lack of distinct
facial pattern, and the two-toned underwings that are diagnostic in
IDing a big falcon as a Gyrfalcon.
We think of the classic Gyrfalcon as white, but most of them that come
our way are dark, like this bird, so it's definitely worth scrutinizing
carefully the dark blobs in trees, or especially on the ground, around
likely areas like Dead Creek if you're hoping to see one.
The most likely misidentification when the bird is in the air is with a
big immature female Peregrine, so look for those underwings if you can
see them, and the really wide wings and bulky chest. And if the bird
banks, that insignificant-looking tail spreads out massively, nearly
reaching the wings.