Welcome to canonify! Where I go through various things in the how to to train your dragon universe, and see if its considered canon! (Please note this is for entertainment purposes and are not official).
According to the official website, it is said gronckles can fly backwards. But it has been never seen in the any franchise sources so one might say non-canon
Right? Well think again! As shown in "Thor Bonecrusher", when Fishlegs explains to the gang that a scauldron's hotwater blasts can burn the scales off a screaming death, Meatlug was flying backwards! Nice save their dreamworks! I claim this fact canonfied!