Originally Posted by Charizard Michelle
Brave would raise attack, lower speed.
Impish would raise Defense, lower Special Attack.
Careful raises Special Defense but lowers Special Attack.
It would depend on what you want to do. From what I know about Gigalith, they have massive attack and defense. Adamant and Brave would do if you want a bit more attack. The rock has so little speed that being a bit underspeed shouldn't matter too much. Getting a bit more defense would be nice and if anything Impish be the best in my opinion if you want a bit more Defense.
A Brave nature would be ideal for a trick room team
Adamant is the best choice but if you can look for a Relaxed , Impish which would help out its great defense.
Before settling think about what you're going to be using it for. also i suggest checking ivs, because even if you do get the right nature if the ivs are bad its a waste to keep it.