Gold Dwarves have always been reputable for their incursion into the Underdark to conquer lost lands, especially with the exploding population they've experienced with the Thunder Blessing. Along with the existing NPC clan of gold dwarves in the Devil's Table (i.e. Clan Gholdring) and gold dwarves often learning Undercommon from their interactions with Underdarker races, it seems reasonable a dissenting gold dwarf would be an outcast and know Undercommon.
Naturally, these gold dwarves should be at minimum non-good.