I do not believe that enough useful GPUs exist in the world, in purchasable form, right now for any entity to take over the network.  It would require a door to door search and confiscation in at least one major western country.

If you wanted to do it with CPUs, it would take about 150,000 quad core Xeon E7450s.  But that is the top end CPU, and it certainly doesn't exist in that quantity worldwide.  Go down a tier, and you need about 300,000 of them.

For reference, google had about 150,000 CPUs back in 2007.  They've probably grown considerably, and might have 300,000 CPUs total by now.  But they will be a mix of CPUs from the last few years, probably none were ever top of the line at time of purchase.  If you operation is massively parallel, you don't need the fastest CPUs, you need the ones that give the best bang for the buck.  And google is using whatever CPUs they have to make money hand over fist already, so it seems unlikely that they would want to shut down their search operations to take on bitcoin in a big way.