Tom's right. You can choose any route you want.
As a baseline, the ADK High Peaks guidebook suggests the most direct route:
Start at the Roostercomb trailhead in Keene Valley. Follow the trail directly to Hedgehog (i.e., no side trip on the Roostercomb spur trail, no side trip to Snow Mountain). Continue over LWJ, UWJ, Armstrong, Gothics, Saddleback, Basin. After Basin, go directly to the junction with the Phelps trail (the main trail from JBL to Marcy), and continue to Marcy summit (i.e., no side trip to Haystack, no extension to Skylight).
That's the most direct route, and the one most people mean by "The Great Range Traverse." But any of the side trips, extensions, etc., are all worthwhile if you've got time and energy. Haystack is particularly tempting, as it so close and so spectacular, and you don't get that far into the wilderness every weekend.
Descent options from Marcy vary also. Many people spot a car (or a bike!) at ADK Loj, for the shorter run out Van H trail. I've usually been with a group that likes to "close the loop," so we head back to JBL and the garden. It's a few miles longer and more descent, but if we're lucky we catch the bus from the garden down.
Have a great hike! The GRT covers some of the most spectacular terrain in the High Peaks.
TCD