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Outlook is a free email client from Microsoft that can be connected to your Gmail account and send out mail merges. To install Google Apps Sync for Outlook, follow the instructions on this page: Microsoft Outlook and Google Apps Sync for Windows
There are Gmail add-ons that provide mail merge functionality. ITS does not support the use of third-party Google add-ons because some plugins are malicious and could steal your data or compromise your computer.
See Microsoft's web site for instructions on Mail Merging to Outlook. https://support.office.com/en-us/article/Use-mail-merge-to-send-personalized-email-messages-to-your-email-address-list-c49e5cfc-53f9-4ec8-9bd9-1f8f3648f1b7