It's not fully a nerf, but a change. Honestly, GX has the highest physical DPS in this game with at least 3-4x higher than the second best class so even with the EDP change he is still way up there as the highest DPS using frenzy.
The difference is that the playstyles will get some change as not only is EDP changed, many skills and interactions are also changed. Example, cross impact will no longer be halved when using EDP so the damage will be retardedly high, and at the same time, Sonic blow bcomes this monster skill that does a lot more damage to monsters with less than half HP (also no halved mod). There will be a lot more variety to the class instead of spamming just RC/CRS and crit with frenzy. I can see cross impact build becoming a lot more popular.
However, for those with JDR, worry not since the no CD 7000% cross ripper slash with EDP and 70% atk JDR will still be king even when EDP becomes x3 instead of 4. IRO have the strongest version of katar GX amongst all server because of the cross implementation of Dogtag with kRO JDR (300 base atk) instead of jRO JDR (140 base atk).