I think most 3-gun guys would never use a grip pod & would usually just have a rifle length handguard & no sling at all. Most 3gun use objects for supported shots and a grip anything would just get in the way of taking supported shots.
For pure standing shots I would use nothing just to keep the weight off. I have a grip pod on my RRA ELITE OPERATOR2 as it is #1 my home defense rifle & #2 my rifle I practice iron sights with out in the field with no rear sand bag and for pure screwing around spray and pray plinking. Other than that grip pods are pretty much worthless IMO. I have a grip pod just so I have the option of having it for that special situation out in the field. My grip pod comes off my rifle more than I use it at the range. If I'm sitting at a bench and I have the time, the harris 6-9 bipod is going on the rifle. If I'm standing taking shots then I'm creating a shelf with left arm or with sling support. Any grippod or grip anything would usually get in the way during a 3gun & most use objects for support when possible.
What is nice about a grip pod is they are easy to take on or off, nice for field use with no rear sand bag (as they sit so high that you can't use a rear sand bag anyway), and screwing around on a bench for plinking shots when 100% accuracy is not needed (spray and pray type stuff)... They are not the best for groups on a bench.
If you do get one, then you can usually pick up the nice steel leg insert GPS-02 off ebay for about $50, maybe $55 shipped brand new. Put it this way, they are nice to have as an option being that they are so easy to take on or off and they are so cheap. As long as you have an available rail on the bottom of your handgaurd. Other than that, for pure standing shots, keep it off for less weight unless you're in a war clearing a room and you need to shoot around a corner or something.
My grip pod is used more as a rifle stand than for actual use. They do have a need for special situations though as this is my home defense rifle. Get it! It is always nice to have and is easy to take off & is fun to screw around with from time to time. They only cost $50 to $55 for a GPS-02 off ebay. They work great for 30round mags as it clears. Reason why I like 20round pmags so much as 30round mags don't clear a 6-9" harris bipod!