There is not a built-in way to send out an email merge from Gmail.
Outlook is a free email client from Microsoft that can be connected to your Gmail account to 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 add-ons because they are not fully supported by Google and also warns that there are known malicious or insecure plugins that 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