Can you help me understand how reflashing the app would help?
Reflashing app effectively copies the app to the phone again, thus ensuring that it is not corrupted. It would only make a difference if the application has somehow changed compared to the original that you're reflashing. My understanding is that the application space is protected in Android. If the GMail app is corrupted then either something maliciously overwrote bits of it (extremely unlikely, and only possible if you're rooted) or the internal flash memory is failing (extremely unlikely). Also, changing bytes in a DEX or any other executable file (thus corrupting it / making it different to the original) would almost certainly prevent it working at all, or at best make it crash frequently unless the attack was highly specific and structured (also extremely unlikely). I don't understand how reflashing it would help, and I don't understand how the symptoms point to a corrupted GMail app.
Don't get me wrong - I'm not trying to knock your suggestion, I just don't understand why you think it would help.