You can easily link an alternate email address to your main Gmail account by going into your "Accounts and Import" settings on desktop or a mobile device. You could also login with another Gmail account for quick swapping between inboxes.
Learn about adding a second Gmail address, or how to add another email address to the Gmail app (even non-Google one) to keep all your emails in one place.
To avoid the hassle of logging into each email account separately, use this helpful guide on how to add another email address to Gmail. It will help you keep all of your emails in one place so you won't have to remember to log in to each account individually.