Urban legend per this article - http://www.todayifoundout.com/index.php/2014/01/seagulls-will-blow-eat-alka-seltzer/
Dunno, I’ve witnessed it. But it is true I did not have a video camera in 1980.
One that is not a myth though, the gull that grabs your bait when you are fishing and you snatch it out of the air