If you are using the Gmail Mobile App on Windows Mobile you may notice that it seems impossible to enter a username and password into the authentication screen.
You can actually enter the data, but you need to do the following:
1) Select the Username box
2) Click the “return” key twice. (should be in the lower right of your QWERTY keyboard, looks like a down arrow)
3) Enter your username. Click Done.
4) Do the same for Password.