This is a great starter video. It gives a broad overview of features that you can use in your work and personal Gmail. It doesn't cover everything but is a great orientation to the some of the most useful tips and tricks in Gmail.
It is a bit long, so shorter videos on more specific topics can be found below.
Some of these were made by Ryan O'Donnell of Rocklin USD, and are used with his permissions. In 2017, Rocklin USD also made the transition from Outlook to Gmail, and they have agreed to let us use some of their materials. He mentions his own school district a few times, but you can ignore that and still learn how to do what is being show in his videos.
Although you are welcome to watch these in any order you like, they were set up to be watched left-to-right, top-to-bottom, and sometimes they will be a little more clear in that order they appear on the page.
Getting Oriented in Edu Domain Gmail
Compose, Reply to, and Forward
Conversation View In Gmail
Inbox Configuration & Basic Settings In Gmail
Attaching Files From Your Computer
Attaching Files From Google Drive
Searching Emails in Gmail
Categories & Labels Instead of Folders
Filters and Filtering In Gmail
Contacts and Contact Groups
Google Hangouts - Chat and Instant Messaging
Adding Themes to Beautify Gmail
Advanced Personalization in Gmail: Themes, Inbox Types, and Labs
for those of you that just feel more comfortable with certain features and set up.
Dealing with Spam in Gmail