I know that is the common view but it doesn't seem to be supported by what the effects of this would be given how gates are constructed to open easily to prevent even minor breakthrough injuries. I understand that many people with years of experience believe that breakthroughs have a negative effect but there is such a thing as selective memory where people tend to be more likely to remember the cases that reinforce their traditional beliefs and interpret the fewer cases they remember that contradict this belief such as, Fio Rita's Whitney, as rare exceptions.
This is not a conscious effort to be biased but just the way the human mind works. Maybe that's why some very good handicapping authors like Quin, Quirin and Sartin have doctorates in Psychology and understand this effect.
Fortunately, controlled scientific Quirin-like studies do not suffer such disadvantages but unfortunately, no such studies exist on this variable to confirm this.
Guess, until then, I'll just go with what the laws of Physics and Physiology say about the effects of such light contact between the horse and the gate.