The credentials requested while installing a gateway are used to setup the cloud services required, assign yourself as its admin, and use the recovery key you provide to generate additional keys to encrypt data source and connection credentials. These credentials are used to get a short-lived access token which isn't stored as opening the local UI prompts to re-login on every launch.
Once installed, the gateway runs in the context of a service account local to the machine where you installed the gateway, which can be changed if required.
You could opt to use a service principal (using the DataGateway PowerShell Cmdlets) as an admin or even add gateway admins to allow multiple users to manage the gateway.