Physical Kinetic Energy (Gigan)
Gigan, in its very debut, establishes a true career highlight as being the first daikaijuin the entire series to draw blood from Godzilla, indicating the incredible sharpness of the claws.
Showa Gigan's mass is 25,000 metric tons.
Its atmospheric flight velocity is actually referenced in the film itself, which is always a nice change of pace from having to rely on profile information: specifically, it's stated to be flying at Mach 4 (1361.11 m/s).
KE = (0.5)*25,000,000*1361.11*1361.11 = 2.31577554e+13 joules or 5.53 kilotons of TNT equivalent.
The result is actually rather low relative to the scale of damage it inflicted on a creature as durable as Godzilla, but its effectiveness can likely be attributed to the highly concentrated area-of-effect of the scythe tips, so no big deal.
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3# - Travel Speed
Gigan and King Ghidorah are both classified as "space daikaiju", as appropriate to their extraterrestrial origins.
However, vs Gigan helps to shed a bit more light on their precise occupation when away from space conquest by revealing an actual homeworld to be typically based on (or at least Gigan's; Ghidorah appeared to simply have bumbled into yet anotheralien race brainwashing scheme while engaging in campaigns of planetary destruction).
Specificially, the homeworld is said to reside within the curiously named Nebula M-Space Hunter (NMSH). With a veryrough timeframe provided in the film itself, as well as a fundamental low-end distance attainable, I'll attempt to discern an exo-atmospheric travel velocity for both Gigan and Ghidorah.
I originally suspected that NMSH was actually the Nebulan alien term for the real M1 Nebula (colloquially referred to as the Crab Nebula), but it appears that I simply misunderstood the actual name of the NMSH (I believed it was "M-1" for whatever reason) and that, most likely, NMSH is entirely fictional. That being said, it shouldn't be entirely unbelievable to establish even a low-end distance from Earth.
Namely, using M42 (the Orion Nebula) as a suitable placeholder to derive a minimum distance: M42 is the nebula in closest proximity to us, at a mere 1340 light-years away.
The Nebulan agents stationed on Earth receive a transmission from their homeworld, informing them that the proposal to summon the daikaiju to assist in conquering the planet had been approved and to prepare for their imminent arrival.
The fact that the Earthbound agents only receive an initial response to their request for aid by the time that the daikaiju had already departed on their objective, along with the explicit necessity for preparations on the agents' end, all lend towards the suggestion that they left NMSH in a relatively recent timeframe, one which rendered consistent relaying of information superfluous. And given the civilisation's capacity for developing superluminal technology in other avenues (spacecraft, Gigan's cybernetics), there's nothing to suggest that their communication isn't occurring in real-time.
They arrive in local space approximately 11 minutes of footage time later, with the Earth not particularly far away.
1) Low-end assumption is 1 Earth day (24 hours).
2) Mid-end assumption is 1 Earth hour (60 minutes).
3) High-end assumption is 11 minutes(treating the footage time as parallel to the in-verse duration and that assuming Gigan/Ghidorah departed only immediately after the HQ transmission).
1340 light-years = 1.2677e+16 km
Low-End
24 hours = 86,400 seconds
1.2667e+16/86,400 = 1.467e+12 km/s or 4,894,200 c
Mid-End
1 hour = 3600 seconds
1.2667e+16/3600 = 3.521e+13 km/s or 117,460,000 c
High-End
11 minutes = 660 seconds
1.2667e+16/660 = 1.921e+14 km/s or 640,700,000 c
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Undoubtedly fast velocities all around; personally, I would subscribe to the low-end as the most 'accurate' result, due to how tenuous my basis for the shorter timeframes are.
Regardless, I should also point out that I don't believe either Gigan or King Ghidorah are capable of attaining these kinds of speeds in real space. I point to the arrival into the Solar System sequence once again, with images in sequential order:
Image #1 is the closest view we get to the dot-like orbs, before they explode energetically and transform into the much larger and more characteristic space-faring cocoons that contain Ghidorah and Gigan. The meteor and crystal cocoons respectively appear to be their 'standard' mode of exo-atmospheric travel though, so either they possess an additional "deep space" travel form for crossing inter-stellar distances or they possess some type of warp capability. Either way, a second mode of travel would go some way to explain the massively superluminal capabilities that have never been seen before with their more typical transportation style.
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Godzilla vs Gigan (1972) Physical Kinetic Energy (Gigan): 5.53 kilotons of TNT equivalent. Travel Speed (Gigan + King Ghidorah, Deep Space): 4,894,200 c [low-end] - 640,700,000 c [high-end]