There are several mail merges applications where you can merge
information from Google:s "Sheets"(GS), but not so may free ones that are
good.
The point using mail merge is that you can personalize your outgoing
mails and you will fore sure get a better response rate.
There should also be said that if you are having a free Gmail
account you are limited to 100 outgoing mails per 24 hours.
The first application, free !, handles the merge via a GS with a built
in JavaScript macro and in the second application, 25 free mails per 24 hours(limit in app), the
merge is done via an installed GS add-on with many nice functions.