Over at the Conglomerate, David Zaring identifies "which American arbitrators are most likely to handle ICSID disputes":
Charles Brower is our iron man, with 6 concluded and 6 pending arbitrations under his belt. He is an emeritus White & Case partner. Michael Reisman (Yale, 4 pending)), and Andreas Lowenfeld (NYU, 2 pending + 2 concluded), looked like our most commonly used professors, though Ronald Cass (BU), David Gantz (Arizona) and David Caron (Berkeley) have also received work, as has Thomas Buergenthal (a professor/international judge), Benjamin Civiletti (a former AG), and Fred Fielding (the White House counsel).
Follow the links at his post for the full list, which is not limited to Americans.
For a WTO comparison, I'll note the following U.S. WTO panelists (just WTO, not GATT), taken from our subscription service:
Davey, William (3)
Echols, Marsha
Hudec, Robert (2)
Hufbauer, Gary
Jackson, John
Janow, Merit
Lavorel, Warren
Powell, Stephen
And, of course, Jim Bacchus and Merit Janow have served on many appeals as Appellate Body Members.