I'd go EMT with compression fittings. A little more resistant to abuse IMO.

Frankly, sched 80 PVC (which is really what you want anyhow) isn't any worse the EMT. EMT can be buckled pretty easily, some PVC will shatter. Note, that there are some restrictions on EMT in corrosive environments (think places like chicken houses, etc...with lots of "manure.").

But to answer the other question, you must keep the current carrying (hot and neutral) wires separate on the protected side of the GFCI, but there is no reason to do so with the grounds. In fact, they will be bonded together without actually passing through the GFCI device.

Q. how do you plan to bring the ground wire around to each outlet if you are not going to use the same wire bundle as hot and neutral?

I intend to have 9 circuits, one for each receptacle all from one main EMT conduit, setting a T or a box to hit each of the receptacles. My question is should I have a separate ground wire for each circuit or can I just share the ground with all nine circuits, thus reducing the number of wires in the conduit.

I wouldn't use thinwall in a barn. It rots right out from the ammonia vapors. Go as nonmetallic as possible. I agree with Ron, schedule 80 pvc is a good choice.

My preference in barns is to keep all wiring high enough that animals, including horses, can't reach it. But I like to keep it in plain sight so that any future damage is visible. Barns often have vermin, including mice and rats, that like to chew.

I don't know whether you have to homerun each receptacle's ground wire if you do so with its current-carrying conductors. However, unless your tees are labeled for volume, and the volume is enough to allow splicing, you can't splice there.

You can always splice in boxes. When determining box fill, remember to count wires that pass through the box or tee unspliced as one fill unit.

EMT is fine in barns, I use it all the time in large horse and cattle facilities, with no signs of corrosion 10+ years later. Probably wouldn't use it in a large enclosed hog barn, but I suspect the OP is not going to use his barn for that. Plus, PVC buckles and warps and generally looks like amateur hour. JMHO. I do second DavidR's suggestion to go with boxes rather than tees at your pipe run intersections. Just easier to work with.

Fairly sure the ground has to go with the conductors.

When you say '9 circuits', that means to me that you have a panel with 9 circuit breakers and 9 sets of NM (or whatever you are using), each going to 9 GFCI receptacles (which seems like overkill). In this case, the grounds are within the NM cable and cannot be strung between the 9 receptacles (like daisy-chaining).

He wouldn't/shouldn't be using NM in a barn. He's talking about running circuits in conduit, in which case he'd be pulling individual conductors for each circuit. Plus, you cannot use NM in conduit, with the exception of specific instances (short spans) to protect from physical damage (one example ... running NM through drilled joists of an unfinished basement, then running down a block wall in conduit to a plug).

1. Nine circuits, each #12 THHN with it's own neutral and a shared ground through EMT with boxes or Schedule 80 PVC with T fittings. One circuit in each bay/stable of the barn and used for storage for boats, cars, whatever that might need a trickle charger. There is no NM in the building.

NM is not a good choice in a barn. Too much moisture and corrosion potential from urine ammonia.

Unless the code has changed, you can still use NMC. Since NMC is almost impossible to find, the standard substitute is UF. Now and then you even see UF dual rated as UF/MNC-B, but that's not required.

UF needs physical protection where it might be subject to damage.

I don't really recommend it, though. It's too easy for small furry critters to gnaw on and damage.

Maybe others haven't, but I personally have seen thinwall badly rusted in barns. The installations had probably been in place longer than 10 years, however.

I still recommend schedule 80 pvc.

There was nothing ever said about using NM. I asked EMT or PVC and a shared ground.

I just threw NM out there as an example, not a suggestion or recommendation.

Okay, gotcha.