From the Operation Overlord Expansion Rule Book:
Overlord in the
The US Marine Corps Commander-in-Chief can let
one, and one only, of his Field Generals order 1 more
unit than indicated on the Section card he gave him
to play. He chooses which one, and tells him so at the
start of the turn, when giving his Command cards out.
All Tactic cards that activate 1 to 4 units activate 2
to 5 instead, even if multiple Field Generals receive
a Tactic card this turn.
When a US Marine Corps Field General or
Commander-in-Chief counter-attacks his Japanese
opponent through the play of a Counter-Attack card,
he orders 1 more unit than indicated on the card
of his opponent.
The reverse is not true and a Japanese General
countering a US Marine Corps Field General that
plays a Move Out card to order 5 units only counterattacks
with 4 of his own, for instance.
The Air Power, Air Sortie, Artillery Bombard,
Barrage, Close Assault, Infantry Assault and Their
Finest Hour cards are all unaffected by the US
Marine Corps Gung-Ho! power.