If I use Gengar's "Hurl into Darkness" attack, can I put a LEGEND Pokemon card (the top or bottom half) in the Lost Zone?
Yes, you can. Each half of a LEGEND Pokemon is treated as 1 Pokemon card.
Source: Triumphant FAQ (2010-11-04), PUI Rules Team (2010-11-04)

Relevant cards

Note: All applicable cards may not be listed below.
HGSS4_94