Open the Mail App:
if it already has email accounts logged in click on Mail on the menu bar, then click Add Account
If it does not have any email accounts associated with it there will be a popup asking you to select who your email account is through (Yahoo, Google, etc. see the screenshot below)
Select Google from the list of options
The following screenshot will then popup
Click Open Safari
Enter your Gmail information in the sign in boxes
Once you have finished entering your email and password the following popup will appear
Select at least: Mail, Contacts, and Calendars, then click Done
Your emails should begin to download now
Check that your setting are the same as the following in Mail’s Preferences