South Shields claim victory in inaugural Jak Fada Memorial Trophy tie with Cleethorpes Town
The game was poised at 1-1 before Gillies' 25-yard strike midway through the second half of a poignant meeting between the 2017 FA Vase finalists.
Cleethorpes had taken the lead within the first 25 seconds when Lewis Collins' superb effort from outside the area found the top corner.
Lee Mason brought Shields level from the penalty spot before half-time and Gillies wrapped up the win after coming off the bench.
The Mariners were playing for the second time in three days, having opened their pre-season programme with a 2-0 defeat to Sunderland on Thursday.
Graham Fenton and Lee Picton made nine changes to the starting line-up and Shields were rocked within moments of kick-off as Collins' strike from 20 yards out on the left nestled in the top corner.
The hosts recovered well from that early blow, with Millenic Alli a bright spark on the left as he made his first start for the club.
He almost helped bring Shields level on 14 minutes when he beat his man on the left of the area before firing a superb ball across the box which Lee Mason was just unable to connect with.
Jon Shaw and Gary Brown were also off-target with efforts, while a further attempt by Alli from the edge of the box was deflected over.
The Mariners were dominant in possession and almost got their reward five minutes before half-time when the ball dropped to Mason 12 yards out, but Owls goalkeeper Lewis Exall made a superb fingertip save to deny him.
From the resulting corner, Shields were awarded a penalty for a foul on Mason and the forward buried the spot-kick into the bottom corner to bring his side level.
The hosts continued to put pressure on Cleethorpes in the early stages of the second half and almost took the lead as an Arron Thompson effort from 20 yards dropped just wide.
Callum Guy also tested Exall with a decent strike, but Shields would have to wait until the 69th minute to get their breakthrough.
Gillies lined up a free-kick from 25 yards and buried it past Exall with a terrific strike. Shields' defence was never likely to be breached from there.
Teenage goalkeeper Aaron Steavens - making his first senior appearance for the Mariners - was called upon to make one save from Jaime Ture, but was not seriously troubled.
Shields continue their pre-season preparations with a home game against a Huddersfield Town development side on Wednesday.
SOUTH SHIELDS: Steavens, Brown (Wright 46), Charlton (Adams 64), Shaw (c) (Morse 71), Wokoma, Thompson, Daly (Lowe 58), Guy, Trialist (Foley 58), Mason (Turnbull 46), Alli (Gillies 58).
Goals: Mason (pen 41), Gillies (69).