The catcher should work fine for this mower, but you do need a bracket. Also, If the opening needs to be bigger you can remove the first baffle (there are 2 at the back of the GCEXXL) to extend the opening by an extra 4 inches. As long as the catcher covers the opening, it should work fine. You can even cut into the rear baffle a bit without hurting the catcher, but I don't think you will need to do this to get good catching performance.
Call our office tomorrow and we will ship you out a free bracket. If you have any problem with the catcher, please let me know and we will be sure you get taken care of.
It does not void the one year warranty to do these small alterations for a good fit, the catcher is designed for it.