Hi DMSlickman,
The Mail app on your Mac acts as a way for you to interface with the account provided by the Email Service Provider (ESP), in this case Google/Gmail. It does so by downloading the information from your account to display to you, and then sending your outgoing messages via their server.
The first step will be to ensure the emails you're missing are still present at the ESP site. Log in and confirm they have not been deleted, moved or are otherwise unavailable for the Mac to download.
Then, follow the steps from Rebuild mailboxes in Mail on Mac "to update the list of messages it contains. For example, if messages appear to be missing or garbled, or if you can’t find all relevant messages when you search using the Entire Message search option.
- In the Mail app
on your Mac, select a mailbox in the sidebar. - Choose Mailbox > Rebuild."
Regards.