how much gain if any
 

I have a 04 silverado 4.8 was wondering if I change out the fan and shout to a newer models electric fan would I gain any hp if so is it noticeable

Theoretically, Yes your gonna gain horsepower, torque and fuel mileage. But it's about as significant as turning off your air conditioning.

Don't do it!
 

There are a lot of race cars out there with electric fans they tend to run off the battery for a short time, but for the street I wouldn't do it; you have to generate the juice to drive the fan and that sucks hp.

When you have a belt driven fan it is driven off the engine at 1:1, but when you generate electricity it takes 200 watts of energy to generate 100 watts of electricity. If you have two fans requiring the same amount of energy, the electric fan will cost you twice as much power to run it over the mechanical.