Alex Simmons on 24 minutes and Craig King eight minutes from time had their efforts cancelled out by a Reece Thompson double in the 41st and 86th minutes.
Ashley Hemmings, Thompson and James Clifton were denied in the shootout, with Gregg Smith on target and it was left to Liam King, Ashley Worsfold and Curtis Bateson to clinch the title for Trinity, despite a Kern Miller miss.
PENALTIES
Ashley Hemmings (BU) - missed (0-0)
Liam King (GT) - scored (1-0)
Reece Thompson (BU) - saved (1-0)
Ashley Worsfold (GT) - scored (2-0)
Gregg Smith (BU) - scored (2-1)
Kern Miller (GT) - missed (2-1)
James Clifton (BU) - saved (2-1)
Curtis Bateson (GT) - scored (3-1)
Gainsborough (3-5-2): Henrich Ravas; Michael Jacklin, Kern Miller, Nathan Stainfield; Craig King, Jordan Richards, Shane Clarke, Liam King, Curtis Bateson; Ashley Worsfold, Alex Simmons (Bradley Wells 80). Subs (not used): Josh Lacey, Tom Davie, Matty Taylor, Ellis Storey.
Boston (4-4-2): George Willis; James Clifton, Jordan Keane, Brad Beatson, Jan Yeomans; Tyrell Waite (Harry Vince 89), Jamie McGuire, Jack Broadhead, Ashley Hemmings; Reece Thompson, Kabongo Tshimanga (Gregg Smith 83). Sub (not used): Karl Hawley.
Referee: Martin Chester.
Attendance: 323.
Photograph courtesy of John Rudkin.