I really hope this is a bug, since she wasn't offering Gofannon frames last week. It's otherwise a pretty scummy move if you're basically telling us we can only play one of the forges per week, Bungie. Especially when that forge is above 95% of the player base's power level.
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Ada-1 only offers certain weapon frames per week. Last week were the Weapons for the Volundr Forge, this week are the Weapons for the Gofannon Forge. Aside from that, you can only forge the Weapons that come from the Quests, so the Hammerhead Machine Gun and the Tatara's Gaze Sniper.