Update Jan 26 2015: Gmail has reenabled it again!
This will be a short post. Gmail seems to have reverted the changes they made three days ago when they started bumping font sizes by over 50% in their iOS apps on the iPhone and iPad.
As of today, the notice below no longer appears in my Gmail app on the iPhone – and this includes old emails as well that used be modified by Gmail.
Let me know if you are still seeing the changes in your Gmail app.