Gerard: as you know it is used as a given name.However, I believe that its use as a surname was in fact an uncounsious error.I have found a marriage record for only one Gerard in 1724 in Quebec (Drouin).No descendant. Could have been an error.
Guerard: From the Red Drouin, I have a marriage of a Guerard as early as 1667 for Martin Guerard dit Gratte, dit Lard, from Normandy.Other dit names (aka) for Guerard include: Le Grapt, Larole, Gratte, Lard.
Girard:Some Girard may have been written as Gerard in English-speakings milieus at times, but not generally.Girard, on the other hand, has also become: Girardin and maybe Girardeau and Girardet.
Hoping that this has clarified the situation somewhat.