It's easy to sign in to multiple Gmail accounts at once. By this way, you don't have to sign out and sign back in again! Follow these steps below to add your accounts or log into different user in Gmail!
Gmail login different user on an Android device
1. Open the Gmail app.
2. Tap 
.
.
3. Tap the down arrow icon.
4. Tap Manage accounts then tap Add Account.
5. Tap Personal (IMAP) or Personal (POP3).
6. Type your email address and tap Next.
7. Type your password then tap Next.
Done! A new different user has been added!
You can switch between your Gmail accounts by tapping and selecting the account you want to switch to.
and selecting the account you want to switch to.
Don't have a Gmail account yet, read this one: Gmail sign up.
Gmail login different user on a Web browser
1. Log into your Gmail account.
2. Click your profile icon at the top right.
3. Click Add account.
4. Type your email address or phone number then click Next.
5. Type your password then click Next.
Done!
To switch between accounts, you can click your profile icon then select the account you want to switch to from the drop-down list.