You might have more than one Gmail account for any personal or business reasons. I will show how you can you manage more than one Gmail account in one browser at the same time.
How to login more than one Gmail account
Login to Gmail account and click on the profile picture.
Now you can click on to Add account.
Here you can you put your id and password to login your another account.
Please Note :
First login Gmail account will be your default account. Which you mean if you will open any other Google app, it will open your default Gmail account.
If you will sign out any of the Gmail accounts then all email account will get logged out.