A tenant is able to manage its own set of gateways. Please note that this feature might be unavailable when the tenant is configured without gateway support.
That a gateway belongs to a given tenant does not mean that the usage of a gateway is limited to the tenant. Every device in the whole network will be able to communicate using the gateway. The tenant will be responsible however for managing the gateway details (e.g. name, location) and will be able to see its statistics.
Gateway statistics are based on the aggregated values sent by the gateway / packet-forwarder. In case no statistics are visible, it could mean that the gateway is incorrectly configured.