Is there a way to delete thousands of gmail emails at once?

yes but only on a computer, the app won't do bulk. search for what you want gone first, e.g. older_than:2y, or from:somenewsletter, or category:promotions. tick the select-all box at the top, and then look just above the list - there's a small link saying select all conversations that match this search. that's the bit everyone misses, without it you only get the 50 on screen. then delete. it sits in trash for 30 days, so empty trash if you're doing this to free up space.