I looked it up in their website and this is what I found:
Please call Hornady at 1-800-338-3220 for ballistics information.
Later, on their website I fond this info:
[FONT=verdana,arial,helvetica,sans-serif]Our engineers [/FONT][FONT=verdana,arial,helvetica,sans-serif]designed the bullet to work with conventional [/FONT][FONT=verdana,arial,helvetica,sans-serif]reloading data, and have the same B.C.'s as the corresponding SST® and INTERBOND™ bullets of the same weight and diameter. [/FONT]
If that's correct then the new GMX has a BC = 0.430