JP server updated their FAQ telling more details about the HP of Gigantix. Seeing from what's listed on the FAQ section of the global site, I guess they might not translate this FAQ entry about Gigantix sooner, so I'm sharing the info here.
Q. How is the HP of Gigantix decided?
A. The HP of Gigantix depends on the number of players around it and changes in real-time. The total number of players in the same section won't affect its HP.
Please note that the FAQs on the JP server actually have some outdated entries which are not correct. But this Gigantix entriy was updated with the emergency update of today, so we can consider this as their latest official statement on the topic.