According to Njáls saga, he was a powerful, athletic man "capable of jumping his own height in full body armour, both back and front".
He was a skilled archer, and in close combat his weapon of choice was the atgeir, which scholars consider to have been a halberd or glaive of some sort.
He was said to have taken this famed weapon in battle from a man named Hallgrímur, while on a Viking raid to the island of Eysýsla (Saaremaa in present-day Estonia - see detailed account on that page).
Initially, Gunnar intended to depart, but when he saw his homestead from the distance, he was so moved by the beauty of it that he changed his mind and decided to remain behind.
When his bowstring broke in close quarters combat, he asked his wife Hallgerður for hair from her head to mend the bow.
Gunnar had slapped her previously, when he discovered his wife had stolen food from a nearby farm during a famine, and she vindictively refused.