>>How did you go ??
With GB lake as a base camp & Center basin as a day-hike, I think he's implying you'd just jump back on the PCT and go over Forester.
Not to derail your plans, but Shepherd is usually taken (when taken at all, that is) to get **immediately** into the Wright lakes basin, and/or Wallace, and/or Whitney zones. It's the pain vs pleasure trade-off. Not to generalize, but it's sort of a local's route into the high Sierra when they don't want to deal with the crowds/permits @ Kearsarge/Whitney.
By going over Kearsarge/Forester and then out via Shepherd, you'll end up doing a lot of backtracking through some pretty dry terrain. After all the effort of Forester, it's just as easy, and possibly more fun, to throw Whitney into the mix, rather than backtrack via Shepherd.
Not only is the shuttle situation easier, but you get to see more high Sierra. The trick with the permits is that the forest service keeps 40% of the Trail crest exit permits as walk-in (ie non-reserved). What happens is they'll check your entry date @ Kearsarge and then jot down the exit date via Whitney portal.
If for some reason you can't get a WP exit permit, then you can always fall back to the Shepherd plan.