Kovalev might be too big for Golovkin, it will be interesting to see how he fares at Super Middleweight when he steps up.
I think GGG would beat him should he move but its too hard to make a real informed guess as we don't know how well GGG would make the transition up through the weights.
If he took his time to move through the weights correctly I would actually favor GGG. He bossed him around sparring (though Kova was still fairly green in fairness) and I think he's still the far more polished technically, even though Kova is criminally seen as just a puncher. The height and reach difference is about the same as when he fought Murray so I don't think that would be such a huge advantage. To me it would just depend if he could carry his power and keep the same speed at LHW, which if done gradually I think he could. Also, I think Kova's chin can be held into question a little. Hopkins managed to stiffen his legs a couple of times when they fought...
I know it's only sparring, but GGG beat Kovalev up quite badly a few years ago when based in Germany. Both have come on since then but it's worth noting.