Gold ore happens to be quite an uncommon find but there is no specific floor of the mines it always spawns in. Your best bet is to go to the lowest floors (highest for upper garmon mines) and mine there. The lower(higher for upper garmon) you go in the mines the better chance you have at getting the more uncommon ore. Mostly because there is more ore to be had the further into the mines you go. If you need gold ore im assuming you have already done the red bell atleast and can visit upper garmon and the floors below where the red bell is. Just keep mining the lower/higher(upper garmon) floors and you will get some. Also some extra advice, dont refine till you have a nice size stack of gold ore, me i refine at around 20 ores normally but since gold has a 5% success rate on refining I would say 50 maybe for all your tools. Don't quote me on that though XD