If you have archived an email in Gmail app and if you want to bring it back then what you do is, go ahead and open gmail. Select the menu option at the top. Select “All mail”. So, if this is the email which I had archived and if I want to bring it back, I will select it. Tap on this menu option at the top and select “Move to inbox”. Now this email will again go back to the primary inbox. So, this is how you can retrieve an archived email in Gmail app. .
As found on Youtube