Wanna help but your issue is a bit confusing. When you say that all emails are going to your gmail, do you mean to say that you're using a gmail application or a gmail email address?
If you're using an application/email client then you might just be seeing all the emails set up to your app/client on one inbox. In this case, have you checked if the emails are really being deleted on your Postoffice (TPG email account). If it is, just look for the settings in your app/client and remember to tick the leave a copy of messages on server to make sure that the downloaded emails on your app will still have a copy on the Postoffice.
If it's the latter might want to check with google.