The send later function is a nice tool to use for anyone that would like to preschedule Mail Merges to be sent in the future. To do this in Gmail all you need to do is:
Step 1: Open a new Mail Merge.
Step 2: Write your Mail Merge as you normally would.
Step 3: Click "Schedule Send" in the bottom left corner of the compose screen.
Step 4: Choose the date and time you would like to schedule the send for and click "Schedule".
The Merge will now be saved in your "Drafts" folder until it is ready to send.