If it's set up as IMAP, delete them in Outlook - it's faster.
In the default app you can delete all.
Otherwise, just click each one you want to delete (the default app has checkboxes, shows more emails at once, so would be better than the gmail app) and delete. You can check all the visible boxes, scroll, check all the visible boxes, etc., and delete 100 at a time.
I feel some of your pain - I had over 500 emails to delete yesterday from one account. (Now that the spammers have that address, the account no longer exists.)