G fare is also a capacity controlled Gov't fare. The Gov't has contracts with AA that they can purchase full fare Y tickets at a reduced rate when traveling on official gov't business.

They now have G fares, which are completely refundable, changeable without a fee, but if you are booked in G, you have to change it to G. The difference in fare is pretty significant depending on the city pair.

Gov't travelers get the same YCA fare if booked in Y regardless of the date you ticket. It is fully refundable. As long as there is a coach seat, you can purchase it.

With G fares, the rules are the same, but the fare is less, but you have to book from the G bucket. If you do it far enough in advance, the gov't saves some money on the ticket.