How do I select a default email account (my gmail) for sending photos directly out of Photos for Mac?

Since Photos doesn't have an email option under the Share button (in Ventura)

you'll need to either: 1 - use the Photo Browser button in the Mail window

or 2 - export the photo to the Desktop and drag and drop it into the email's window where you want it.

With #1 you may have to search for the photo by file name or title or add a unique keyword to the photo, search by keyword, and then remove the keyword from the photo.