Leeds pipped defending champions York to second place in the league on points difference to gain home advantage for the semi-finals.
But Valkyrie dominated the opening quarter and were ahead when Gentles twisted over for her first try of the season.
Georgie Hetherington almost added a second, but knocked-on as she crashed over from close range, and York had Carrie Roberts to thank for preserving the 6-0 lead at half-time with a try-saving tackle on Olivia Whitehead.
Andrade powered over straight after the break, but Ebony Stead sped past the York defence to cross and Keara Bennett’s goal cut the deficit to 12-6 with 13 minutes left.
Valkyrie had Tamzin Renouf sin-binned in the 74th minute for late contact, and Leeds had a chance to level when 2024 Woman of Steel nominee Lucy Murray touched down with two minutes left.
But Bennett could not land the touchline conversion and York held on to reach their third successive Grand Final.
