Scheduling emails to be sent at specific times can be tough. Especially if you’re not at the desktop when you need to send the email and since a function isn’t built into Gmail itself.
Frankly when something so popularly desired isn’t available, you can expect someone to make a plugin, or an addon for Gmail. Email providers by default should have the ability to schedule email, but sadly don’t which is quite odd. It can be added using a 3rd party as shown below.