Ian Grieve walks through creating a macro via mail merge in his new post
Microsoft Dynamics GP Macros: Macro By MailMerge
I will note that a detailed explanation of this is also present in our various Dynamics GP Cookbooks.
Microsoft Dynamics GP Macros: Macro By MailMerge
Archived post from 2017. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.