To use Gyaku-Zuki during kunite requires a lot of speed and quick reactions. The fist has to cross a longer distances than a Kizami-Zuki.
In order to train it fast repetitions of Gyaku-Zukis are prefereable. Keigo Shimizu shows in the following video how to do it. Thank to Rene Winkler the video is in slowmotion. Thus, you can see Keigos posture during punches.