In Administration > Users and Permissions > User Management, you can edit any individual user. Within the User’s record is a built-in Manager field, which you can populate with any other Gainsight user using type-ahead.

You can also sync this value in from SFDC (or other data sources). For SFDC, you can add the Manager in to a Rule or into a Connector job. This is especially helpful if you have the Manager recorded and updated consistently in some other system, so you’re not forever trying to keep up with manager changes.