TGV reservations

TGVs require a seat reservation. You can’t stand, like in regional transportation. So they could be sold out. And you’ll pay a higher price purchasing tickets on the day of travel.

The answer to your specific question is yes.

I did exactly that last summer, flew the overnight non-stop flight from SFO to CDG, landed ca. 11 AM , dilly-dallied a bit in CDG, took the RER to Paris Nord to have a leisurely lunch at Paris Est, bought TGV ticket (reservation comes with it) from the SNCF machine using the credit card the tap function (I could easily have paid cash also) to Metz, where I aimed to arrive at by 4-5 PM just in time for my reserved check-in after walking 5 mins from the train station to the hotel, a 3 star for 70 Euro.

I knew going to Metz last minute on the TGV would be no problem, never encountered one before either.

If your train happens to be sold out, then take another departure...,only a slight inconvenience.

"Can I walk off a plane at CDG and buy at TGV ticket to Avignon without a reservation?"

Yes, you can buy a ticket but note that the cheaper less flexible tickets may be sold out for purchases made on the date of travel. TGV trains can sell out, but as mentioned above, take the next available one. I've had to do that for trains to Avignon. There is a music festival in Avignon in the summer every year so it is possible that if you are traveling during that time period, I think there is an increased possibility that some trains will be sold out.

I would venture to bet that trains to Avignon are slightly more in demand than trains to Metz.

Looking right now I see that for immediate departure most trains today still have seats, but some only in 1st class. So you could certainly wing it. There are plenty of trains on the route. Worst case you might have to change trains in Lyon or Valence.

It also depends when the OP is traveling. Seats available for walkup on a Monday in March ? Great ! In July ? Doubtful.

are pre-purchased tickets changeable or refundable?

You can purchase changeable / refundable tickets for a higher fare. You can do a dummy booking on SNCF or Trainline to see how much the difference might be.