In Gmail you can use the operator in: attached to a mailbox name to show only messages in that box and I find using in:trash in the search box much easier than digging for it in the controls on the Gmail page. Once Trash is shown, there is a link at the top of the page that reads Empty trash now which does precisely what it says.
In Google Drive, there is a control Trash (with a capital T) that when activated will show your trash folder. Once that folder is actually showing, there is another dropdown control, labeled Trash as well, that when activated gives you the option to Empty Trash. [It's also possible to restore an item or items in trash to their former locations, just like Recycle Bin in Windows.]
Brian - Windows 10 Home, 64-Bit, Version 1809, Build 17763
Explanations exist; they have existed for all time; there is always a well-known solution to every human problem — neat, plausible, and wrong.
~ H.L. Mencken, AKA The Sage of Baltimore