For the second consecutive season, the Blades suffered an opening day defeat in Sky Bet League One.
In the first competitive game of Nigel Adkins' reign, United were disappointing as Gillingham netted three of their four goals from set-pieces.
The confirmed team news an hour before kick-off saw Long given the nod to start the season opener at Gillingham. The homegrown youngster played behind one of his former Academy team-mates, McFadzean, who was handed the left-back berth. New boys Sharp and Woolford also started, but Sammon had to be content with a place on the bench.
In the Kent sunshine, the Blades wore the new macaw green change strip but were on the back foot for the opening ten minutes - Gillingham won two corners and lumped into two long throws, from the second of which they opened the scoring. Jackson provided the big throw and it was OSADEBE who was adjudged to have forced the ball over the line at the near post amongst a cluster of bodies.
There was another nervous moment on nine minutes when an interchange between Collins and Long saw the latter's clearance charged down by Dack, before United went on the offensive. As the home side backed off, Basham almost got in and Adams attempted to nick the ball from the arms of Nelson before Sharp's first chance on 18 minutes, but the ball trickled wide.
On 28 minutes, an Adams run was blocked and as United appealed for a free-kick, Gillingham broke away and Williamson's shot was deflected away for a corner, where Oshilaja won two headers, the second of which was cleared off the line by Sharp.
For United, Freeman fired into the sidenetting and for the home side Dack curled over from distance as the game ebbed and flowed close to the break - before more woe. An unnecessarily conceded corner resulted in OSHILAJA forcing the ball home from close range, again in a crowded penalty area.
The Blades made a brighter start to the second period. Sammon replaced Baxter at the interval, which meant positional changes for Adams and Woolford, who delivered a corner from which Basham went close from an acute angle.
Gillingham remained an attacking threat, though. Dack's free-kick saw Long parry and Oshilaja was wide with the rebound, moments later Norris should have done better with a back post header.
Reed and Scougall were introduced with little over 20 minutes remaining as United still sought a way back into the game. On 76 minutes Murphy broke through the middle and shot from distance, but Nelson pushed away, whilst United's penalty appeals were waved away when the Scottish winger fell under Jackson's challenge.
United's hopes, however, were dashed with a third Gillingham goal on 82 minutes. Another set-piece saw a corner delivered from the right and the goal was credited to EGAN in another crowded box. Moments later Egan met another corner but headed over.
To sum up United's day, Murphy, clear on goal, watched his shot blocked by Nelson's legs and Basham was crowded out by Dickenson as the goal gaped... and then, deep into injury-time, DACK cut in from the left and beat Long with an effort which found the corner of the net.
Gills: Nelson, Jackson, Egan, Houghton, Williamson, A Morris (Hessenthaler 45), Osadebe (Wright 89), Norris, Dack, Dickenson, Oshilaja.
Unused: Garmston, Ehmer, McGlashan, G Morris, Donnelly.
Blades: Long, Freeman, Basham, Murphy, Sharp, Baxter (Sammon 45), Collins, Woolford (Reed 69), McEveley, McFadzean, Adams (Scougall 69).
Unused: Howard, J Wallace, Higdon, K Wallace.