When you run a migration for an Office 365 group, you are not migrating the group itself, but the owners and members respectively into a group within your destination site.
In order for group owners and members to be populated correctly at the destination, you must create a mapping between the source and destination groups. ShareGate Desktop will map groups automatically only when promoting a subsite to a site collection, or when the groups have the same name. This is by design, since in many cases, users want to preserve the groups as they are on the source.
Note: If you find that a user is missing, then it is often the case that the user does not exist at the destination. You should see warnings to that effect in your migration report.