I’m not @azulryu , but I’ve made a Gyarados list in the past (very outdated and contains some mistakes unfortunately, since I made it mid-2017). @shizzlemetimbers may have an up-to-date list, since he collects English and Japanese Gyarados as well.
As for your questions regarding the Topps variants available, I recently posted this here. Gyarados was in the third set, so should be available as non-Holo; Rays; Flames/Teardrops; Balls/Cross-hearts; Sandpaper; Squares and Rectangles; Dents; Smooth Silver; and Smooth Rainbow. And those are just the holofoil variations, because there are also blue logo, black logo, green logo, red logo, and circle Nintendo logo variations of most of these. And there is another Gyarados in the second Chrome set, which should be available as Chrome Standard; Chrome Spectre; Chrome Sparkle; and Chrome Tekno. Some more information about Topps holofoil variations, which is where I got most of this info from, is from this Topps article from @samsamsamsmsms (he might perhaps have pictures of each Gyarados, since he collects Topps cards).
Hope that helps, and good luck with your collection goals! 
Greetz,
Quuador