The Gyarados (XY60) card can be found in the Xy Black Star Promos set.
Recently sold 3 times and based on those sales is worth around £28.61.
Currently, there are 2 open listings for this card on ebay.
This card sells in the range of £19.18 to £38.04, making it quite stable.
This Promo card was illustrated by TOKIYA.