The GPM upgrade process applies database migrations that go beyond normal application-level changes — creating filegroups, partition functions, and partition schemes, and adjusting database-level settings such as the recovery model. These are database administration operations, not just data changes, so they require permissions beyond the db_owner role that GPM normally runs under day-to-day.
Because of this, the account (or DSN) used to run the upgrade must be a member of the SQL Server sysadmin role. This is only required for the duration of the upgrade — GPM's regular runtime service account does not need sysadmin and can continue running with db_owner.
Note: don't confuse this with the separate requirement that the MonitorIT Server Service account be a Windows administrator on the SQL Server. That's an ongoing, day-to-day requirement (unrelated to sysadmin) covering the service's normal operation. The sysadmin requirement above applies only to whoever/whatever runs the upgrade itself.