However, in gen1 water moves were special. Gyarados has atk of 125 and special of 100 in gen1. This means that hyper beam would do slightly more damage(~1%) than hydro pump.
I see, Pokemon showdown is assuming that you get the get the stat increases from battling to 100. My calculation is without these stat increases. Like leveling with rare candy. Those 64 points make the difference.
In gen 2 they split special into satk and sdef. Gyarados has 60 satk and 125 atk. Hyper beam being a normal move uses atk in gens 1-3. Hydro pump just gets worse. I doubt the way they changed stat calculations makes up for 65 attack power. Looking at this in Pokemon showdown I think there is something going on with these damage calcs. My guess is damage has a cap somewhere and these moves are hitting it before weakness/resistance multipliers are applied.
Just in case anyone is curious it seems base damage in the formula is capped at 999 before modifiers. Later all damage is lowered to the defenders max hp if the damage is higher. Pokemon showdown seems to do the former but not the latter.
It's not. Censorship has reportedly stopped the author from continuing the story. However, they do keep uploading side stories and art of the characters periodically. I'm not sure Twitter is better than Pixiv but I'll put their Twitter link here.
Foom Rule (slrpnk.net)
Kiss [SQ: Begin W/Your Name!] (beehaw.org)
Source: 坛九 on Pixiv