yes, for example luffy and the others use them to fight shiki's pirate. but some people put haki on their guns, in bullets or arrow so the power is very strong than the usual guns.
There are guns in all versions of One Piece except the 4kids English dub of it. Either you haven't watched any One Piece at all, or you've been watching 4kids, because guns make frequent appearances in both the anime and manga.
Still , the straw hats almost never fight with guns. And guns are only frequently used in the whitebeard war saga
Also, aside from the 4Kids dub, at the very beginning of the series a gun is first used in chapter 1 and episode 4. Where Lucky Roo, from the Red-Haired Pirates, shoots a guy in the head. And then Benn Beckman takes out the other, like 50, bandits using his rifle as a "club".