If you didn't watch that video Gavin plays with a black undershirt hanging below his jersey... been doing this since his freshman year. This is NOT unique to Gavin... many QB's do the same, around here that is.
Also... nearly every player you see in a football game today... very visible or hardly noticeable, makes some type of unique 'fashion statement' when out on the gridiron. What is legal?
In my day you better be perfectly uniform with your teammates or you either corrected it immediately or you left the field... I saw many players tossed to the sideline until they corrected their uniform faux-pas.
Today I see a rainbow color of socks on the field. Some jersey's tucked, some not. Some bare-armed some with underclothing with sleeves.
I know there does exist 'rules' on what can be worn on the field but I have NO IDEA what the rule(s) says. Whatever the rule says it's obviously very lenient OR the officials have just chosen to ignore that rule.