Go to settings, applications, manage applications, all tab, gmail, clear data. Notice the "low on space" icon on top now gone.
The issue appeared right after I installed Firefox on my HTC Incredible, this app is almost 20 megs. Uninstalling it, however didn't do anything. It seems that this "low space" issue is chronic on HTC Incredible. I had the same issue a month or so ago. HTC, if you are reading this please attend to this problem or face consequence of loosing to IPhone 5.