There is a short list of receivers who stand under 5'8 that have had any success in the NFL.
The list isn't very long, either.
Dating back over the past decade, only five players under 5'8 have posted 50 or more receptions in their career. Four of those players have been successful in the past three seasons. The fifth player is one of the best returnmen of all time: Dante Hall.
The four players with recent success are the Browns' Andrew Hawkins (recently fought over during free agency by the Browns and Bengals); the Cowboys' Cole Beasley; the Jaguars' Ace Sanders; and the Texans' Mike Thomas. None of these players are world beaters, but they all have had important roles as slot receivers (Hawkins, Beasley, Thomas), or as a human jackknife (Sanders).
From a film perspective, Gallon seems like he'd fit into the Patriots offense. He was asked to line-up inside on the slot, but also spent plenty of time as an outside threat. He ran a wide variety of routes and always seemed to find his way into a hole in the opposing zone, or found a way to shake out of man coverage.
He generally plucks the ball out of the air and seems to fit well with quarterback Tom Brady's tendency to throw out routes low and away. He plays larger than his size and has plenty of body control. There's little to say that he's a better prospect that Kenbrell Thompkins or Josh Boyce, but he'll fit right in alongside them.
From a statistical standpoint, he has a chance to be the best out of all the aforementioned players. I reviewed how each sub-5'8 player performed against ranked teams or in bowl games over their final two seasons in college (Andrew Hawkins didn't have any data points, so I compared how he fared against Bowl-division teams).
Gallon stood out as the best of them all.
Hawkins averaged 2.2 touches for 5.2 yards and 0 touchdowns over his five qualifying games at Toledo.
Beasley averaged 5.0 touches for 55.0 yards and 0 touchdowns over his six games at Southern Methodist.
Sanders averaged 4.1 touches for 49.0 yards and 0.5 touchdowns over his eight games at South Carolina.
Thomas averaged 9.4 touches for 91.5 yards and 0.5 touchdowns over his eight games at Arizona.
Gallon averaged 7.4 touches for 112.8 yards and 0.8 touchdowns over his eight games at Michigan.
Note that this does not include Gallon's 14 receptions, 369 yards, and 2 touchdowns against Indiana.
It's fair to say that Gallon elevates his play against premier talent and I wouldn't bet against him having similar success in the NFL. He plays with speed and finesse, yet is strong enough to fight for the ball. He looks to have a strong grasp on how to attack defenses, and has the athletic ability to follow through.
Making the roster won't be a given for Gallon- but it's a tall order that I think he can handle.