As the OP has discovered, there's a big difference between measuring components in-circuit and out of circuit. Measuring parts in-circuit requires you to know what exactly that circuit is, or at the very least for repair purposes, a known working unit of the one you're working on to compare it to.
AndyC's point is that GFX cards are remarkably complex, and measuring resistance between two random nodes, on a non-passive circuit designed to switch three figure watts without know exactly what that circuit is is somewhat futile.
As you have already removed the devices in question, you can at least measure these.
It's going to be hard to clean up those pads and solder on parts (either the originals if they are fine, or new ones) because those boards are made for heat transfer, and so may require preheating.
I've only repaired two graphic cards in my time, and both times it was a painful experience.