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The recommended option is to use the technical user. In this case, you do not have to take care of the user roles assignment in the partition.
I would like to grant Gitlab access to the IntegrationManager instance to a user who doesn’t have a Gitlab account. Is it possible?
Yes, it works the way that if the user doesn't have a Gitlab account yet, Gitlab sends them the invitation via email when they are granted access to some IntegrationManager in PlatformManager.