Regigigas is actually a pretty revolutionary card! It boosts the maximum HP of non-EX Basic Pokémon to what used to be the maximum HP of, well, Pokémon-EX, back when they were first introduced. It's an increase of 30 HP compared to Zygarde BUS, which not too long ago set the record itself.
And another cool thing! As I mentioned in the Discord chat, you can Level-Up this Gigas to Regigigas LV.X SF100 and lose 30 HP in doing so––which, I believe, is the most it is currently possible to lose upon a forward-shifting change (evolution, level-up, etc), beating out the previous record holder of Kabuto's 80 -> Kabutops' 60. (You can further this somewhat by attaching a Fighting Fury Belt to Regigigas, causing it to lose 70 HP upon Level-Up.)