Anyone know of an after market air filter for increase power,throttle response for the GX470 Lexus 2005

K & N makes on for about $60-70. I noticed better performance on my GX by installing it.

Does the K&N air filter affect gas mileage at all?

K&N, like all the orher so call performance drop in panel filter does nothing for horse power or gas mileage. About the only difference is that the engine will feel quicker due to the new filter and it is louder. To increase horsepower noticebly, you need a K&N cold air intake kit.

Yeah, it's not a huge difference, but you can feel the engine "breathe" better. If you want to see some big differences, yes, the cold air intake is the way to go.