ChrisJMV
Hey. Is it possible to mount the H110i GTX in the top in the case?

There should be no issue the case fully supports 280 MM rads

Is there any manual or so available for the case to see how to mount it in the top? Currently using a Corsair 780T but want to change

it doesn't show in detail how to install but the image below is the best view we currently have. basically you install the radiator on the underside of the mounting brackets and the fans on the top side... i dont know exactly how this works though.
 

Yes I just mounted my yesterday. 
Note that I mounted the rad on top, and fans inside the case proper. I DID HAVE AN ISSUE...
 
When installing the radiator, all the way to the (now right side, formally the front) you will have TWO issues:
1. The top cover will not close all the way prevent the power buttons from working. 
2. Depending on the MB, the hoses are too short may not reach.
 
The correction is to loosen the fans and slide the rad as far back towards center as possible.
This allows the top to clear the rad. The reason the top does not clear otherwise is that it angles down at the sides. I still have pressure on the memory sticks ( with their fancy high cooling fins) and the hoses are stretched tight. So be sure to check your memory stick clearance. 
 
Next step would be be to use the more rearward fan slots on the top to more center the rad and loosen up those hoses. This might be preferable ANYWAY as you cannot even see the right most fan as it is hide behind the EVGA cover panel. ( Looks like I have just one fan up there).  I am not using lighted fans, so I do not care, but if you want to SEE both baths... better center up that AIO. 
 
Cheers 
Pine

The radiator is 27mm thick.
Fans on top or Rad on top, buyers preference, one could argue. I placed it as I did, in case at some point in the future I did want lighted fans for that extra "eye candy". The question was asked and I attempted to answer as honestly as I could, given that I have just installed that exact unit.  As you correctly state there is plenty of room inside the case even for the rad and the fans. Doing that and bring the rad either more centered or to the back/left, would let the viewer see the full effect of the rad and fans. However, then there are the wires and the rather drab grey of the rad side, I prefer to hide that part.