Blake Orban, Alec Baer and Cain Franson opened up a 3-0 lead in the first period for the Giants (3-7-2), who are tied for last in the Western Conference.
The Hitmen (9-3-2), who lead the Eastern Conference, forced extra time with goals from Jake Virtanen, Calder Brooks and Elliott Peterson.
Vancouver goaltender Payton Lee finished with 32 saves, while Calgary's Chris Driedger stopped 29 shots.
WINTERHAWKS 5 TIGERS 3
At Medicine Hat, Alta., the Winterhawks scored three unanswered goals in the second period to beat the Tigers.
Chase De Leo, Keegan Iverson, Nicolas Petan, Taylor Leier and Brendan Leipsic each scored as Portland (9-3-1) earned its sixth straight win.
Steven Owre, Blake Penner and Cole Sanford had singles for Medicine Hat (7-3-2).
Winterhawks goaltender Brendan Burke made 40 saves, while Marek Langhamer allowed four goals on 34 shots in net for the Tigers.
OIL KINGS 3 WARRIORS 1
At Moose Jaw, Sask., the Oil Kings overcame an early deficit.
Brandon Baddock, Edgars Kulda and Riley Kieser had a goal apiece for Edmonton (7-6-1).
Brayden Point scored the lone goal for the Warriors (5-7-3) just 35 seconds into the game.
Tristan Jarry finished with 28 saves for Edmonton, while Justin Paulic stopped 37 shots for Moose Jaw.
RAIDERS 3 PATS 1
At Regina, Prince Albert broke a 1-1 tie with a pair of goals in the third period.
Jayden Hart and Josh Morrissey each scored in the third, while Dylan Busenius also had a single for the Raiders (8-5-1).
Boston Leier scored the lone goal for the Pats (7-6-0).
Nick McBride was spectacular in net for the Raiders with 42 saves, while Mac Engel was less effective for Regina with 17 saves.