A daggerlike instrument attached to a gun and named after city in France?

You're likely thinking of a Bayonet.

While not directly named after a city, the bayonet's name is derived from Bayonne, a city in southwestern France. The name is believed to have come from the fact that early versions of the bayonet were produced in Bayonne.

It's a dagger-like weapon attached to the barrel of a firearm, primarily used for close-quarters combat.