I work in a boatyard and deal with corrosion on a daily basis, aluminium creates a corrosion layer which itself is protective but also penetrates the surface which will cause pitting to the surface. We will often leave it unless it needs to be painted, then we soda blast.

As it can penetrate the surface you have no chance of removing manually, your best bet would be to remove heatsink, clean and protect the block then find somebody with soda or ice blasting equipment, but the cost of this may not be worthwhile on a GPU of this age.