How do you schedule an email to send later in Gmail?

Yes, built in. Compose your email, then instead of Send, tap the small arrow next to Send > Schedule send > pick a date and time. It sits in a Scheduled folder until then, and you can cancel or edit up until it sends. Works on both the web and the Gmail mobile app.

On mobile it is the three-dot menu in the compose screen > Schedule send. Same feature, slightly different spot than desktop.

Scheduled emails live in a Scheduled label on the left. Open one and cancel send to pull it back to a draft before the time hits.