In 2002, for the revival of Oklahoma!, the Gershwin was reduced from 1933 seats to 1670, partly due to a wall being built behind row J of the Rear Mezz (other seats were lost in the front of the Orchestra due to the Oklahoma thrust stage).
When Wicked first opened the Gershwin Capacity was 1773. During the first year of Wicked's run, (beginning with Week Ending August 8, 2004) the center wall was removed and the center section of the Rear Mezz (Rows K, L, M) was added back to the Gershwin inventory bringing it to 1809.
In 2013, just after the 10th Anniversary, they opened up the house right and house left walls in the rear mezz, bringing the Gershwin back up to its current full capacity of about 1926. The last 4 rows of the rear mezz Right and Left sections total 119 seats. So it seems when they expect sales to be lower on a slow week, they take the last 4 rows offline to lower their capacity/tighten their inventory in order to capitalize on the dynamic pricing structure now in play.
Similar to when the Rear Mezz curtain is closed at the Broadway, limiting its capacity from 1740 (West Side Story) to 1480 (Dr. Zhivago) and the St. James when they close the Balcony, bringing its capacity from 1684 (Frozen) to 1303 (Side Show).