Double-check your Gmail IMAP settings within Mail to ensure that Mail's version of your Gmail account's Trash folder is actually mapped to the Trash label on the Gmail servers:
1. In Mail's left-most pane (where the mail folders and IMAP accounts are listed), show the entire contents of your Gmail's account structure. If the Trash folder is still listed there, it is not mapped to the Trash folder "on your Mac" for Gmail.
2. Select the Trash folder under the Gmail account
3. Under the "Mailbox" menu item, click on the "Use this mailbox for..." option and select Trash
4. The trash folder under your Gmail account structure will seem to disappear. In actuality, the contents of that folder will now be displayed in the Trash mailbox up the pane. If done correctly, that mailbox should now have a selection triangle next to it and when opened will reveal the Gmail Trash folder in it.
What all this does is ensure that when you delete the contents of that Trash mailbox, the contents of the Gmail account's Trash will also be marked for deletion (or immediately purged, depending up on you have that set up with Gmail itself).
FYI, you can also do this with the Sent, Drafts, and Junk/Spam folders for any IMAP account. This ensures that the actions you take in Mail will be synched to the appropriate mailboxes/folders in your other IMAP accounts.