Is the Gigabyte R9 290 Windforce OC same as the REFERENCE PCB? Planning to hook it up with a Kraken G10

nope

almost none of the "special" gpus use the same "reference" board as the original card. if you plan to replace the cooler get a reference card don't get a "windforce".

well i did say "most" as for the windforce i don't know. These "special" editions almost always are modified from the standard setup in some way. However that's not always the case. For example, Powercolor and Asus almost never modify the card. I do not know about the windforce 290 though.

let me know if it is. i'd like to know the answer to this too.

The design of the non-reference boards for the purposes of compatibility with the G10 will not be a problem. The mounting holes are almost always exactly the same, as the GPU and shim area are always the same size, socket mounting holes have really no reason to be different, plus the G10 has adjustable mounting holes on the backplate.

Given you are just fitting a water block to the card and the G10 comes with no other components, even if its a custom PCB there wont be any issue regarding the placement of the VRM or Ram modules. Even aftermarket VRM sinks like the Gelid Enhancement kits are probably going to fit, but are actually unnecessary unless you over volt the card for an overclock. Things you are likely to see different on your custom PCB is additional power components located around the VRM area, which will extend the board length and may inhibit an out of the box R9 290 VRM heatsink (of which there is only one, the Gelid), however many users don't have one at all or jerry rig something from smaller VRM sinks from other kits anyway as the Gelid is difficult to get a hold of.

One issue you can come across however on a custom PCB is that the custom branded cooling solution may have a contact area different from the reference model, therefore the height of the GPU die itself might not be totally level with the outer shim on the PCB. This means you will need a copper GPU shim to go over the GPU to ensure it makes contact with the flat base of the waterblock. You can buy the 7970 shim as this fits (generally due to its size) and it can be had from places like FrozenCPU for about 5 dollars.

On a side note the mounting process of the G10 and an AIO liquid cooler is really fiddly as it's design isn't actually that great once you examine it closely. So I probably wouldn't recommend doing it if you are a total newbie, but the instructions are really good, just be careful and take your time lining things up and making sure the bard is insulated from the mounting bolts correctly (newer versions come with nuts and washers instead of rubber stand offs, which is actually harder to fit).

I would suggest checking if you need one or not before installing and running the card, as if the waterblock doesn't make direct contact with the GPU, its going to get really hot really fast and probably not even boot to windows before it overheats and it might not be clear what is wrong. You can just do it by eye or pop a thin layer of paste on the GPU and place a card across it and see if any sticks.

All you really need is for the mounting holes to be the same on the PCB as they are on reference cards, which I would dare to say is almost always the case (aside from perhaps a GTX 760 mini or something of that nature).