From Longman Dictionary of Contemporary Englishgassygas‧sy /ˈɡæsi/ adjective British English DFDa gassy drink has too much gas in it → fizzy This beer is really gassy.
Examples from the Corpus
gassy• Goodbye to your sweaty nights, broiling days, gassy ale and loudspeakers never turned down.• In the early stages of cold-welding the planets must have been very similar in appearance, with massive outer gassy atmospheres.• In some pubs fake hand pumps serve gassy beer.• He drew back his hands and smelled the gassy foulness.• As it approaches the sun it starts to evaporate, giving off the familiar long gassy tail.• It was too fizzy and too gassy to drink and I acquired a taste for real ale. 