There are a few different ways that you can sign into another Gmail account. The easiest way is to simply go to www.gmail.com and enter the email address and password for the account that you want to sign into. If you're already signed into a Gmail account, you can click the "Add account" button on the top right hand side of the page to sign into another account. Finally, if you have multiple Gmail accounts, you can use the "Switch account" button on the top right hand side of the page to easily switch between accounts.