The England Under-19 international - Spurs Ladies' first England representative - scored the fourth and fifth goals on her first appearance as Spurs went from 3-0 up to 3-2, 4-2 then 4-3 before finally coming home 5-3 at Chatham Town FC.
They are now in the last four of the competition they were runners-up in last season alongside Charlton, who beat them the final and Herne Bay. West Ham face Phoenix Sports in the last of the quarter-finals.
Still smarting from Sunday's 4-0 home loss against Portsmouth in the FA Women's Premier League on Sunday, Spurs kicked-off against FAWPL South East high-fliers Gills determined to put right that result and performance.
And it was there for all to see as an own goal (on 13 minutes), Wendy Martin (18) and Katie O'Leary (44) fired them into a 3-0 lead by half-time.
But things started to change after the break as two goals in as many minutes (51 and 52) brought Gillingham back into the game.
Loomes responded with a fine low strike for 4-2 (65), Gillingham replied again within a minute for 4-3 but Loomes had the final word with the fifth and clincher 10 minutes from time.
"First of all, it was vitally important for us to win this game after what happened against Portsmouth on Sunday," reflected manager Karen Hills.
"We started well, took the game to them and looked comfortable at half-time but a couple of errors allowed Gillingham back into it. To the players' credit, they always found a response and Lucy was excellent on her debut.
"Lucy just underlined the sense of belief she has as a player. She works tirelessly across the front and she's not afraid to put her foot in either. She's had a positive impact and gelled so quickly with the team.
"We made it tougher than it needed to be but we got the result and in the cup, that's all that really matters."
Spurs Ladies v Gillingham: Barbara Holman, Mollie Kmita, Alex Keown (Rosie Kmita 46), Jenna Schillaci, Sophie McLean, Shannon Moloney, Maya Vio (Bianca Baptiste 20), Megan Wynne, Katie O'Leary, Wendy Martin, Lucy Loomes. Substitute (unsed): Kelley Blanchflower.
Spurs Ladies are back in cup action on Sunday with a trip to Canvey Island to take on C&K Basildon in the League Cup (2pm).
AVILLA IN NI ACTION
Avilla Bergin is in the Northern Ireland squad for a UEFA Euro 2017 qualifier against Switzerland in Lurgan this evening (Friday, November 27).