Submitted for review: The context of the latter part really supports why 'gil' is just... gill. The kaiju name's 'gil' uses the same spelling as the generic 'gil' as related to fish, etc.
This one ain't very deep. "Gills+evil."
The monster's name appears to be a combination of 'gill' and mara (魔羅) a buhddist demon; a 'gilled-demon' with an oni-like nose.
This may have been a topical reference because at the time Zone Fighter was being produced the American Bluegill fish was causing an ecological crisis in Japan and wiping out native freshwater species. (We swear, it was a gift!)