Well, im pretty new to hydraulics myself, but i know a little.
For one thing, the speed that the grapple operates at is not purely based on hydraulic flow. It's also based on geometry of the articulating linkage and the sizing of the cylinder or cylinders that will operate it. Any kind of hydraulic system can be made to move something quickly, but it is a tradeoff of speed vs power.
Power or strength may not be much of an issue with a grapple, depending on how you are going to use it. For example, in any case where the grapple can reach 'past' or around an object and then close, the grapple is just restricting the space by which the object could slip out, and doesn't really need to 'squeeze' very hard. Once you let off the controls, the grapple will be held in place with MUCH more strength than what it actually used to pinch down. However, if you are trying to grab something with smooth parallel surfaces like a big paver stone or something like that, the actual pinching/squeezing strength might come into play. If you are mostly using the grapple on brush/logs etc I would venture to say that the actual squeezing strength of the grapple is not very important and you can basically design it to attain your desired speed instead. Basically, if you are mostly holding something up with the floor of the bucket and just using the grapple to keep it from tipping/rolling out, strength won't matter. If you are trying to point your bucket straight down and keep a smooth object pinched between the grapple teeth and the floor of the bucket to keep it from falling out, strength will matter.
IF you divert your existing hydraulic flow from your bucket cylinders, for example, you don't need to know GPM because you can just figure out what you need from the size of the cylinders you already have and how long it takes to extend/retract those cylinders right now (preferably in the direction where it's dumping weight instead of lifting it, although the difference would be small). You can calculate the fluid volume of a hydraulic cylinder fairly accurately just by knowing the cylinder diameter and the rod diameter. Compare that to the internal volume of a different cylinder, and whatever percent smaller that volume is the percent faster it will operate, or vice versa, on your existing hydraulic circuit.
The valve you are talking about is called a hydraulic diverter valve, i believe. If you used a diverter valve that was electrically operated (easiest, most common i believe) you wouldn't actually install a 3rd hydraulic lever on your dash. You would probably just install a momentary switch on your bucket curl lever and whenever you were holding that button down the lever would operate the grapple instead of curling the bucket.
