Hitchin deservedly won the game, taking the lead in the 24th minute.
Stewart King was fouled just outside the box and Frendo curled the ball into the top left-hand corner of the net past a stranded Gounet.
Five minutes later the homesters doubled their lead.
Striker Frendo headed in from two yards; Gregson helping the ball on to the Hitchin striker from a Donnelly corner.
Development Squad manager Jon De Souza made one change at half-time, replacing Alfie Mawson with Michael Kamou.
It was pre-planned as the Berkshire youngster was eased back into playing after picking up a knee injury on the German tour.
Despite a better start to the half, Brentford could not create any real chances of note.
Tyrell Miller-Rodney compounded a bad night for the Bees in the 59th minute when he was dismissed midway through the second half for an over the ball tackle.
Line-up: Gounet; Clarke, Pierre (sub Tomasevic 77 mins), Mawson (sub Kamou 46 mins), Griffiths; Breeze, Miller-Rodney, Tavernier, Oyeleke; Norris (sub Moore (M) 84 mins), Caulker (sub 68 mins Onovwigun).
Sub not used: Taylor.
Attendance: 123.