The bill shape is ok for ICGU, but also a good match for THGU, and too small for either GWGU or GLGU, and therefor hybrid GWGUxGLGU too.

Pale bill base evident here, ok for ICGU, GWGU.

Flat head ok for GLGU, male GWGU. Would be rounder for THGU and especially ICGU.

Pale pink legs ok for GWGU and GLGU; usually brighter pink for ICGU and THGU.

Whiter head is abnormal for any first year gull that have brownish body feathers. All of the species in consideration would develop white heads normally after about a year of age. This gull appears to be juvenal.