You can setup your Gmail account in Blackberry through your data plan service provider or by following the steps below.

step 1: Goto the  Setup folder.
step 2: Search for email settings 
Step 3: There you will get an option to add account or you can go through settings and search for Create user name
Step 4: By clicking on Add account you will get a list of Email service providers
Step 5: Select the appropriate option eg: gmail
Step 6: Enter your credentials properly.
Step 7: After clicking on next you will get a display that Account has been added successfully.

Your gmail account has been added successfully.