Suppose that I have a Gourgeist with the "Gourgantic" Ability, with a Grass Energy and Heavy Boots attached and a Training Center in play. Would its max HP be 200 or 250?
Gourgantic essentially replaces Gourgeist's printed HP, so reductions and additions to HP work as normal. You can stack the additional HP effects of Heavy Boots and Training Center, so in this case the resulting HP would be 250.
Gourgantic