GPS relies on timing - very precise timing - and signed signals. It might be that GPS units ignore that the signal should be signed, but the (picosecond) timing basically defines an objects’ position in space. A picosecond makes a difference of a few centimeters.

Now, modern planes don’t primarily rely on GPS. They have gyroscopes. But as gyroscopes lose precision over the duration of the flight, they cross-reference with GPS to fix this loss of precision. But for that, the measured GPS location must be close enough to the gyroscope-based location, or the GPS result is discarded as erroneous. So one needs not only to spoof any GPS signal, it must be close enough to the actual position, and then slowly move the target over.

BTW, the villains in the movie “Tomorrow never dies” use a different approach. They influence the GPS satellites directly, which is a totally different thing, and if Iran did attempt that, I think the US would react differently and … more directly.