Most gull identifications are tied to age, so let's begin by establishing that. The all-brown scapulars, dark-solid tertials, and relatively neat upperwing coverts all belong to a juvenile bird. The pointed primary tips and mostly black bill also aid in ageing this individual.
We don't have any reliable references for size comparisons in the photo, but this appears to be a medium sized gull (smaller than a Herring Gull). The attenuated look to the wings helps narrow it down to a few species, but much more telling is the strong "cinnamon" color throughout which is only shown by two North American gull species: juvenile Ring-billed and juvenile California Gull (much more common on the latter). The bleached appearance to the chest, neck and forehead, as well as the straight tubular bill and decurved gape expression make this a pretty solid California Gull.
This plumage aspect is not uncommon among juvenile California Gulls - loosely named "Cinnamon Types". Photographed in mid-September in San Mateo County, California.