It is a type of knife, much bigger than other knives. Without hilt, this weapons looks just like today’s kitchen knives. Gully’s blade was single edged. Pirates used this weapon for many things on ship, like for cutting rigging and ropes. Gullies were also often used during mutinies.
Pirate dagger is a type of blade that is little bit smaller than classical Knife. Daggers usually have both sides of their blade edged. The daggers have cross bars and hilts in beginning of the blade, which adds a more protection and prevents hand from slipping to the blade. In fights, they meant to be trusted at an opponent. Pirates favored it as a weapon of surprise attack as they could be easily conceal underneath the clothes and quickly slaughter opponent. Pirates found many other uses for daggers like cutting ropes and their food and almost every one wore it.
Dirk is a smaller type of dagger. It was used mainly as a throwing weapon and had either double edged or single edged blade. Generally, shape of dirk has changed a little in the last 500 years.