The former Odsal Sedbergh winger touched down twice to match the record set in 1994-95 by Nick Pinkney, who scored 19 tries in eight games.
Gabriel’s brace took his tally to 12 touchdowns in eight games but he was eclipsed by a four-try haul from club captain James Feather.
Keighley’s attentions will now turn to this weekend’s promotion shoot-out against leaders Oldham at Whitebank Stadium, where victory would take Paul March’s side straight back to the Championship at the first attempt.
James Woodburn-Hall put Hemel ahead but Feather replied for the hosts. Feather grabbed his second after more good work from Barnes before Brendon Rawlins got in on the act when he crossed from close range. Gabriel touched down Paul Handforth’s kick for his first try in the right corner before Hemel replied on the stroke of half-time through Nathan Darby.
Gabriel took a long pass to dive over for his second in the right corner but Hemel refused to lie down and hit back through Simon Hrbek – only for Feather to complete his hat-trick after Josh Lynam had scored.
Scott Law grabbed a quickfire brace before Feather pounced for his fourth try, with Rawlins’s second and scores from Handforth and Jozh Guzdek completing the rout, while Danny Lawton kicked eight goals on his 100th appearance for the club.
Keighley: Guzdek, Gabriel, Lawton, Barnes, White, Mitchell, Handforth, Law, Feather, Bailey, Pursglove, Rawlins, Dickinson. Substitutes: Lindsay, Cherryholme, Lynam, Lee
Hemel Stags: Olpherts, McNamara, Brown, Cousinie, Hrbek, Woodburn-Hall, Coleman, Stewart, Lawrence, Ross, O’Callaghan, Boyd-Barnes, Ljazouli. Substitutes: Muroy, Howitt, Darby, Long
Referee: T Grant (RFL).