Spoiler:

After leaving the rest of his tribe, Gef had to face the rest of the world mostly by himself. Things were hard for him right from the start considering how gullible dwarves traditionally are and in order to survive the world outside his tribe, he had to change that nature through sheer willpower.

However, the gullible nature of dwarves is hard ingrained in the way they're raised from a young age and is hard to teach out when they're older. Gef is still gullible because of that, but has learned to recognize when he's been tricked and responds aggressively towards it; even aggressive enough that people end up dead.

Idolizing the knights from the middle ages, Gef left his tribe at a young age in order to pursue the dreams of becoming a knight himself. In order to gain on that dream quicker, he donned a set of armor to bolster himself against the world and has since taken it off only sparingly. It has helped to keep him alive in spite of people taking advantage of his gullible nature and has assisted in him continuously raze the ground building a path and his new crew.

The brutal behavior that he took on in order to punish those that take advantage of his gullibility has earned him the nickname "Little Berserker" and is a name that rings in the ears of pirates heading to the New World. Once having earned the admiration of the World Government, he has now earned their ire having turned against them in the same manner that he has others. They took advantage of his crew and his benevolence, and now he treats them with violence, earning the title of a Supernova with his antics against them.