First make sure you are connected to internet. Try to use Google crome in which GMAIL will work the best.
Thanks.________________________________________________________________________If it didnt work let me know.
I am able to review my G-mail using userid and password XXXXX google.
However, on my phone when I click g-mail I still get the above error.
Try this link and use this site to use Gmail on mobile.
Which phone are you using?
Try turn off the phone, take out the Battery, this will reset the phone. Try this and see if it helps.
No need to keep the battery out for long. I will opt out so other experts can handle the issue.