Add an account to Gmail

When you open the Gmail app, when you open up and then you will read her, and you can send messages from all your email addresses, even if they are not a Google Account. If you do not have gmail account, you should download the latest version of the Gmail application, please follow the steps below.
Add an account
When you are using a gmail this already, but you do want to log in with a different Gmail, but you also do not want to log out the Gmail account that you used. When it is you're adding more .As an account then you go follow the steps below
1.  First you open the Gmail app.
2. At the top left, you tap the Menu icon.
3 .At the top, go to the right of your user name and hit the down arrow.
4. Click Add Account.
5. And then you enter the account you want to add to that

Google: Gmail or Google for a work address.
Personal (POP / IMAP): An email address that is not from Google, like Yahoo or Hotmail.
If a business address that uses the Microsoft Exchange server. If you are unsure if your company uses Exchange, check with your IT department. This option is available for Nexus devices and some Android 5.0+ others run

6. You follow the above to add your account.
If you ever encounter a problem adding e-mail addresses that are not from Gmail, you can check for other instructions on the support site of your email provider. Here are tips to help you perform a Google search for the word "POP" or "IMAP" along with the name of your email provider
To switch between accounts
  The message on the Gmail app it is in a separate account for each individual differently. To switch between accounts to be able to see the message:

• Please Open the Gmail app.
• At the top left, touch the Menu icon Menu.
• Next to your email address, tap the down arrow.

• Please cick o account you want to use.

See more : 

2 hidden ways to get more from your Gmail address