It could be a few things, each of which are related to the installation of the new fuel sending unit. The sending unit mounts to your gas tank through a rather large hole that is recessed in the top of the tank. There is a seal that is installed between the sending unit and the tank which is usually held in place by a large metal ring. I think that the seal between the sending unit and the gas tank is damaged, allowing a small amount of fuel to leak from the top of the tank when you fill it. This fuel is sitting in the recess at the top of the tank until it evaporates, and that is the source of the smell.
Answer: Your vent and fuel lines also attach to fittings that are an integral part of the sending unit. It is possible that one of these lines was damaged during the replacement of the sending unit, resulting in a small fuel leak and therefore a strong odor of gasoline.
I think that the source of the leak and the smell will be obvious if you remove the gas tank and inspect the seal and the lines around the sending unit. Just don’t smoke while you are doing it.