i thought gpu blocks didnt have an orientation. atleast the ones i have had havent. an EKWB GTX 970 1 and an EKWB RX 480. neither had any instructions that even mentioned what direction if should flow.
All EKWB blocks, CPU and GPU have an orientation. For GPU blocks the port closest to the back plate is usually the Inlet. Although, according to EK themselves the GPU temperature difference is only about 1-2C if you invert it (friend had it "backwards" and contacted them). If you look at the GPU block you should see a plate over the fins. When you use the proper inlet this creates more pressure over the fins since it restricts the opening over them. If you invert it then the water is flowing over the fins and then restricts on the way out. Not as effective but not a big deal either.