You could probably use CGEventCreate to create a gesture event. The event types "officially" defined for CGEventCreate don't include gesture event types, but you could pass in the values defined in NSEvent.h:
NSEventTypeGesture
NSEventTypeMagnify
NSEventTypeSwipe
NSEventTypeRotate
NSEventTypeBeginGesture
NSEventTypeEndGesture
The values for non-gesture types seem to map directly to the kCGEvent<TYPE> values in CGEventTypes.h so it's reasonable to expect the gesture event types will work:
CGEventSourceRef eventSource = CGEventSourceCreate(kCGEventSourceStateCombinedSessionState);
CGEventRef event = CGEventCreate(eventSource);
CGEventSetType(event, NSEventTypeMagnify);
//continue to set up the event