It has Gigabit Ethernet interfaces that can connect to a 1 Gbps WAN circuit.
However the appliance is not capable of forwarding 1 Gbps of traffic. In an ideal test you might get 500 Mbps; but typically it will be lower than that - especially if you are using the Firepower service module.
As per the document it supports only 500MB
But it got Gbics to connect to WAN Switch.