Two quadrate bones are preserved (specimens AMNH FARB 30659 and 30660). It is very straight in general with only a gentle curvature on the posterior border. The articular surface of the head is triangle-shaped and flatted to the lateral side. A relatively large, ridge-like formation can be observed on the posterior edge near the top, likely supporting the squamosal. This character is also reported in other iguanodontians and hadrosauroids, such as Brachylophosaurus or Gryposaurus. The lateral surface of the quadrate is flat and on the anterior border a cut for the quadratojugal can be identified, however, most of its articular borders are lost except a small dorsal area and a long bottom surface. A right dentary is represented by specimen AMNH FARB 30654, a partial element with a badly eroded dental battery. It preserves about 15 alveoli of which none is filled with teeth. The total dentary teeth count on Gilmoreosaurus was probably less than 30 and the tooth row is oriented to the lateral sides as seen in other hadrosauroids, unlike the more advanced hadrosaurids. The upper toothless border of the dentary and a significant portion of the articular border for the predentary are missing, but given that both of these borders have minor lengths—about less than the 25% of the length of the dental battery—they indicate that the toothless area between the tooth row and the predentary was rather reduced—another trait present in hadrosauroids but lost in hadrosaurids. The predentary is represented by a complete element of specimen AMNH FARB 6369, which has a U-shaped form. Numerous and pointed denticles are preserved on the dorsal surface of the predentary, giving a serrated texture.
