Pedantic as I am, I prefer to call them gravity bolts. Beam implies more of a two-way transmission. 'sort of the reason I cringe when people say "laser beam." Of course in that context beam flows more easily off the tongue than would ray. (Also of course, I don't make up the studio's glossary. Sou ka.)
Exploding due to lessened gravity is absurd. Exploding due to absurd tidal stresses is quite reasonable. With that kind of innately powerful attack it has always been a shame that Ghidorah is down-rated so the good guys can win.
Gravitic manipulation also explains why Ghidorah can fly, hover, break orbit, and cruise through space. Spacedragons are versatile critters.