Personally I think that a lot of people are just over exaggerating the nerf. The most they will do is nerf the Wolf Pack Rounds (as they have said they would only nerf) by about 100 damage. Gjallarhorn's Wolf Pack Rounds currently do around 283 damage. This nerf is decreasing the damage mainly so people can think of using other guns instead of just going straight to Gjallarhorn.
Gjallarhorn is still going to be a top tier, if not the best, heavy weapon in the game. It's also to stop the "Gjallarhorn's Only." Which I will be glad to see disappear.
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Even then it won't be then most damaging heavy once ToK comes out because we won't be able to upgrade year one content anymore(to the next standard) so it will forever be a 365 weapon (but no complain that Xur sells it brilliant)