You'll need some distilled water, some folks add kill coils or silver additive to kill any microbes.
Plan on filling the loop, checkinjg for leaks and any last minute changes BEFORE adding any color to the loop, otherwise it will be wasted, which will get pricey fast.
For a GPU you'll need either a full cover block ( and a back plate is highly recommended) or a universal water block. Th uni may require extra cooling for VRM or memory chips, but you can re-use the universal blocks. For additional GPUs you'll need an SLI/crossfire bridge of some kind. Each card will need additional fittings. I recommend getting extra fittings anyway. Barbs are cheap but may require clamps. Extra tubing won't hurt if its your first loop, I redid my loop...