It won't ever happen again. Some ppl have suggested having xur sell it for a larger amount of strange coins but at this point ppl already have a large amount of strange coins from grinding the heroic strikes each week. I have been playing since day one and still don't have a gally. I will admit it that it blows but I'm not banking on xur selling it again to get it or complain that he won't sell it again. Just gotta keep doing nightfall and raids and it will eventually drop