Katie O'Leary led the way with two goals and Leah Rawle scored the third in a one-sided affair at Ware FC.
Spurs now take on Charlton Athletic in the final - date and venue to be confirmed.
It's the team's first cup final since winning the London Cup in 2012 and just reward for an impressive season so far.
The teams met in the FA Women's Premier League South in November when Spurs stormed into a 3-0 lead in the first half and a repeat looked on the cards as the home team poured forward again.
Gillingham goalkeeper Imogen Alabaster denied Wendy Martin and Lucio Leon, Wendy and Avilla Bergin had efforts cleared off the line and Spurs had two goals disallowed before Katie broke through two minutes before half-time.
The team was straight back on the front foot in the second half as Alabaster thwarted Wendy again but then a blow as Shannon Maloney was forced off injured, Riana-Devi Soobadoo her replacement.
Katie headed home from a corner for 2-0 on 69 minutes and after Lucia and Andra Marcoci went close, Leah Rawle expertly tucked away the third goal six minutes from time.
Spurs Ladies: Toni-Anne Wayne; Sophie McLean, Jenna Schillaci, Alex Keown, Lucia Leon; Wendy Martin, Shannon Maloney (Riana Soobadoo), Maya Via; Bianca Baptiste, Katie O'Leary (Leah Rawle), Avilla Bergin (Andra Marcoci). Unused sub: Laura Picton.