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It appears the translator confused the syllable 'tsu' (つ) from 'Getsuga' with the sukuon (っ) that only means the next consonant is repeated (so, 'Gekka').
They missed the dakuten on the ka as well. It's not as if GT is a new move, so when you see the kanji for it, you really should be almost autopiloting into the standard translation rather than something new.
Seeing some chatter that the guy who translated this chapter doesn't read Bleach, which might go ways to explaining it. Pretty much every other translation was spot-on though which is weird if that was the case.