I spent 40 minutes proof-reading a detailed post, only to have my server decide it didn't like being regulated through a firewall. 'Cute.
Lightning creates heat through induction. However, for years now we've been told that Ghidorans spit gravity bolts. Toho approves.
Gravitic stresses could easily cause explosive effects such as Ghidoran bolts create, and have the added advantage of being essentially unblockable. While GKG is notably beam-resistant (having stood up well to even Mothra's OutOfTheButt zaps), even diamond-coated armor is worth exactly jack against gravity-based damage. (The same goes for any forcefield that's not itself based on gravitic effects.)
I treat this as a series of solo fights. GKG flies pretty well (he's Ghidoran, after all), so he's got all the positional advantages. Gamera is arguably the weakest of the defending trio, and flies the best out of them. Mana beam may be a frightening attack, but it's hard to aim and requires Gamera to open his guts to attack by something three times beam-crazier than Heisei Godzilla. 'Not exactly a bright move...
An intelligent GKG would take down Gamera first to guarantee air control. Then, SMG would be taken down hard because it is, after all, intended to fight Godzillas, not Ghidorans.
The outcome of this match settles on whether GKG lands and grapples or beam-duels Godzilla. Godzi's regeneration takes time, and GKG has the noted resistance to beams which, arguably, gives GKG a similar endurance. GKG also out-beams Godzi, which would give him the advantage.
On the other hand, GKG may be able to sever skyscrapers with his wings, but he's more vulnerable to physical damage than to beams, so Godzi could conceivably beat him down, IF the uchakryu was foolish enough to surrender his aerial advantage.