If your Android already has Gmail app, login will be quick and easy.
STEP 1: Tap the “Gmail” icon to launch Gmail.

STEP 2: Open the ≡ menu, then select “Settings”.

STEP 3: Tap “Add Account”. The “Set up email” screen will appear.

STEP 4 : Select “Google” and then tap “Next”.

STEP 5: Enter your gmail username,password and then tap “Next”.
You’ll notice a “Forgot password” link right under the password blank. If you don’t remember your password, tap that link and follow its prompts.