Deleting every email from the gmail app on my phone

the phone app can't do it in one go, it only selects what's loaded on screen. do it from a browser instead: gmail on the web, go to All Mail, tick the box at the top, then click the 'select all conversations' line that appears above the list, then the bin icon. takes a while if you've got years of it. everything sits in the bin for 30 days after, so empty that too if you're after the storage back.