It's just a tad bit more complicated than that

If you wanted to put something down a specific chimmney, then yes.

But they'd only need to get accuracy within a kilometre to do some serious damage. Heck, probably not even that. It wouldn't be that complicated to glue GPS, an altimeter, and possibly some inertial sensors together with a small embedded PC. I'd even bet that a suitable PIC microcontroller would do the job.

The real technical issue though is making sure that it works as expected, and that the code doesn't have any bugs. That's one vehicle that they'd want to ensure went the right way. Obviously they'd also need to protect the system from the elements - ICBM are basically orbital in nature, temperature and radiation are both factors.

Fortunately for us though - civilian GPS has a height and speed limit, something like 40000ft and 550mph, although don't quote me on that (Derrick would probably know). ICBMs would exceed both, and anything slower and lower could be taken out fairly easily I would guess. But that wouldn't rule out the bad guys getting hold of a milirary GPS receiver and using it. I believe that the military signal is encrypted somehow. Obviously, I wouldn't know the details of that. I'm guessing the units need to be manually keyed to decrypt the stream, and that the keys change on a fairly frequently basis. Assuming there is only one encrypted stream, and that there are many listeners, key management would be a weak point in the system. (Somehow every listener would need to change keys at the same time, giving an avenue of attack.)