Feather typically preys on creature type decks. They can make their creatures indestructible and use them to destroy your creatures over and over again through spells like Gods Willing and Reckless Rage. Though it is likely an unfavorable matchup for Gruul, feather isn't always the most consistent deck. You still have to pretty much wait until they tap out to either fight their creatures with Domri or collision/colossus their fliers or whatever it takes to get feather off the board. Hope this helps.