While attempting to set up a Gmail account on a Kindle Fire HD 7″, the system requested an Exchange Server Name and optionally, a Domain Name. I do not know where or how to get that information.
Try using @googlemail.com instead of @gmail.com to see any improvements.
If your e-mail provider is not recognized automatically by the Email app, you'll need to enter your password, log-in name, POP3 or IMAP port, and server settings manually.
Just go through Amazon support website. Here, you can find best guide for setting up Gmail account on a Kindle Fire HD 7.