I upgraded to a Lumia 920 (from Samsung Focus) last week and last night recalled that I needed to do this additional step on my WP7.x/iPhone such that Gmail will actually delete emails instead of archiving (why does Google think that I want to archive junk/spam by default is beyond me).
To Google's credit, they've had this page, m.google.com/sync that you have to open on a mobile device and toggle archive/delete there. It seems that this page has now conveniently stopped working from any mobile device and instead links to a useless page which refers to Windows Mobile
I finally managed to work around it when I found this page
and decided to document/rant here for easy reference.
1. Open IE10 dev. tools (press F12) on your PC
2. In dev. tools menubar Tools > Change user agent string > Windows Phone 7* or Apple Safari (iPad)
3. Open this URL: https://m.google.com...=mobileproducts
4. Now log in and you will be on the old page where you can toggle the archive option.
5. Additionally you can also select to sync additional calendars here.
*I tried first with WP8 user agent but then I reached a different page. Do no evil indeed, eh?