I can only assume from the little that I can find that this app is no longer functional at least for Windows 8.1 phone users.
I switched my main email address to Gmail & bought Metromail when I got my first smartphone, so maybe 3 or 4 years ago. The app has been pretty good in that time, maybe it didn't do everything that Gmail on my laptop will do but it's been useful enough.
Recently I have been unable to open mail; it appears on the phone but I get "error" when trying to open them. Effectively I can send but not receive.
Today I got fed up & uninstalled the app, thinking maybe my phone had a glitch. On re-installing, I can't get to Gmail. When the app first loads you get a column of buttons for various mail options but clicking Gmail just gets to an "error" message & hangs on a blank screen.
It is a real shame, as this was a much recommended, mostly well functioning app as far as I was concerned. I will try something else as I want to keep using Gmail on all my devices. I hate having to keep swapping email addresses etc. it is a PITA.
It seems the developer (Apoelin?) can no longer justify the work involved in keeping the app going for all (or any?) platforms, struggling with Microsoft's systems. I do not blame him. I paid a very small amount of money & got 4 years use. That's damn good value for money unless you think that digital content should be free, in which case I say; if it's that easy, go & write your own program!
Microsoft have almost certainly not helped here. When will all these huge corporations realise that users will not be tied to just one system (theirs)? Why make it difficult for the best Gmail app for Windows phone to work?