If you need to find information from an email that is not in your inbox (ie it has been archived), the search function looks for email addresses and message contents. To search, tap the magnifying glass icon on the top row of the Gmail app. This will open a search box.
The Gmail app will provide predictions of what you may be searching for as you continue to type. If you are entering an email address, Gmail will find matching addresses. Selecting one of these will find all emails from that address. If you enter general text, Gmail will return all emails that contain that text.