Guerdat, on Nino Des Buissonets, was flawless in both rounds of the final.
Silver and bronze were settled by a jump-off as Gerco Schroder of the Netherlands, on London, beat Irishman Cian O'Connor on Blue Lloyd 12.
The Irish rider, who was stripped of his gold medal in Athens after his horse failed a dope test, was a late call-up for the team after rival Denis Lynch was disqualified at a Nations Cup event when his horse was found to be hypersensitive.
In his second round, Nick Skelton on Big Star incurred his first penalty of the Games to finish joint-fifth in North Greenwich.
British team-mate Scott Brash (Hello Sanctos) - who like Skelton had achieved a clear first round - and Mexico's Alberto Michan (Rosalia La Silla) also finished on four penalties.
GB's Ben Maher was ninth after a nervy final round on Tripple X.
But it was a successful Games for British showjumping after their team gold, much of which was attributed to the 54-year-old Skelton's otherwise perfect scores.