The replacement is usually 1mm for VRAM and VRM (if you choose to go with thermal pads). You can (as an alternative) use thermal paste for everything as well (MX-4 seems to be the commoner for that) without issue.

Next week I'm going to repaste my CPU and GPU. I couldn't find an scheme for my GPU GTX870m (G750JS), so I can't decide whether I should use 0.5 or 1mm pad thickness. Apparently 1mm should be Ok right?

1mm will be just fine. The GPU heatsink assemblies ASUS designed have the appropriate rises to accommodate for a specific thickness.

When you remove the heatsink assembly off the GPU, just take notice on where the thermal paste has been applied so you can duplicate it again where needed.

Also make sure that when the heatsink assembly is tightened back down, that you visually look around it to make sure there aren't any obvious gaps near the VRM and VRAM, especially where your thermals pads have been applied.

You can also get away with just thermal paste on everything without any issues because of how the rises are formed into the base plate surrounding the copper for adequate contact when compression fit (tightened down).

If there was thermal pads, you need to use thermal pads, thermal compound will not provide proper heat conduction for the ram/vrm. If you are not sure on the thickness it would be better to go on the thicker side, provided that the pads are all the same thickness. otherwise some parts may not contact the heatsink properly.