Ok, never built a bare frame, so I have no clue on this.
Have: GTB frame. nothing else.

What dictates which headset/fork/stem I can use? (concerning size and also threaded v threadless)
I kinda need threadless, cause i want to switch bars without untaping often (removeable faceplate).
What makes a bike 1in vs 1 1/8... is it frame, or headset/fork?
Any suggestions for a carbon, with brake hole, track geo. fork?

The headset standard is determined by the inner diameter of the headtube. 30.2mm = 1", 34mm = 1-1/8", 37mm = 1-1/4". Since the standards are about 3mm apart you don't need to measure with a precision instrument.

If the inside of the tube is basically a plain cylindrical tube it uses a standard type of headset, or there are cup like areas machined in the ends that's for an integrated or "hidden" headset, but you'll have to go to the LBS to know which type fits.

You buy the fork to match the headset diameter, and need a steerer long enough for the length of the headtube, plus the height of the headset for a threaded headset, and add the height of the stem and any spacers you plan on using for a threadless headset.

There are more details involved in actually fitting the headset, and given your inexperience, you'll probably need help doing so, either from a knowledgeable fellow rider or the LBS, but at least this is a guide to what you have and will need.

Like Francis said. On the removeable faceplate, they come in both threaded and threadless type.