Including an extra d6 to every of your attacks at 6th level is a nice reward to your Rage damage, and scales to 2d6 when you get to 14th level. To my know-how, the one official source could be the single image in Volo’s (Until there’s A different firbolg artwork https://dnd-human60246.worldblogged.com/33969190/the-definitive-guide-to-female-rock-gnome-dnd