I'm sure others will jump in.......
First, a "B" is stamped on the back of the cylinder head. I am assuming this is the piston size that I need to order?
Yes....allways a good idea to have the bore checked.
Second, is it as simple as replacing the piston, rings, gaskets or do I need to consider a replate, bearing change, etc?
Yes, doing a topend it pretty straight forward, wrist pin bearing, make sure you lube it with some premix before install, you don't want it dry. Get all the proper gaskets, the base gaskets come in different thickness, so make sure you get the correct gasket. Reed valve block gaskets as well, however not required to be removed for a topend job. But, I would remove and inspect and replace if the reeds are flaking on the edges, or not closing all the way. Inspect the bore, being such a new bike, one would not think that it would need to be replated, but you know.
Third, I noticed that the Power Valve is completely coated in carbon. Should I just pull it out and clean it with contact/carb cleaner?
Yes, check those little bearing in the power valve area. They should rotate freely and smooth. Clean or replace as needed. Take the p/v flaps out of the cylinder, and clean them. I use a wire brush with cleaner, to remove all carbon, I strive to get a polish like surface to prevent and carbon from building up. Once the bearings are cleaned up, put all back together and the operation should be very smooth and fluid. If there are any "rough" spots, in the movement of the flaps / valves figure out where fix it. It should be like "butter".....as my son would say
Fourth, do it all myself or take it to a GasGas expert 2.5 hours away?
Only you can answer that question....however there is a ton of great guys here on this site that can and will help you out. A topend is easy if you've done it before, but can be a little over the top for some. Having the proper tools, and space to do it makes things much easier. My approach to bike maintenace is much like a doctor, clean tools, clean work space, clean shop towels, all layed out ready to perform an operation. Being clean, neat and organized makes any job that much easier.....but, that me!
Any other tips or suggestions?
Yes, take your time, and prepare.......meaning, perform searches on this site using "key words" and you will find an answer to your questions. Print out the thread, that way you have the info out in your work area when performing your tasks, or drag the lap top out to the shop if you have WIFI? I have a binder full of threads from this site on broken down by parts of the bike, engine, frame, suspenion, ect, ect, comes in handy. Better than any OEM manual.
Good Luck, have fun and when in doudt....ASK!