They both share a few things in common like tons of tight low end, stiff feel, very defined articulation.
They differ in that the GK100 still has that Celestion grit, albeit much less than most other Celestions. As most are aware, the EVs don't really have any grit at all - much more neutral and 'round' sounding than the grindy GK100. Both are good for maximizing the articulation of your rig but I'd choose the GK100 because it has more personality (angry mids) than the EV (though much less than other Celestions).
The highs on the GK100 extend a little higher than other Celestions, so it'll get a bit closer to the EVs clean hifi sparkle as well. Lastly the GK100 is a heavy speaker but the EVs are still much heavier. Important if you're thinking about a 4x12 setup.