All Ethernets are the same in this regard. Read IEEE Standard 802.3 starting with Clause 12 to see the pattern.
In half duplex, there is no ambiguity. Obviously, the bit rate is the specified one, in each direction. But you can only transmit in one direction at a time.
When you move up to full duplex, the "This GMII supports 1000 Mb/s operation through its eight bit wide (octet wide) transmit and receive paths."
Both the transmit and the receive paths provide 1000 Mb/s. And again, consider that half duplex 1000BASE-T exists, and the the signaling (physical layer) of half duplex and full duplex is identical. There should be no major ambiguity here.
It is 1Gb/s in each direction on a full duplex system.
I presume it was a marketing person who decided to call that a 2Gb/s system.
If you are out driving, and see a sign "speed limit 40 miles/hour". I presume you wouldn't decide that since it is a two way road that you could then drive at 80mi/h, and if you did you wouldn't convince the police or a judge.