...you will always get a ghastly out of it, so... technically, you cant ever breed a gengar from an egg, use whatever has better IVs, or if its a male, better attacks --- Shut up, you stupid HIPPIE!! I'll make you listen to DEATH METAL!
You can breed it with anything in the "Indeterminate" egg group (see list here: http://bulbapedia.bulbagarden.net/wiki/Indeterminate_%28egg_group%29), which includes Gastly, Haunter, and Gengar. The egg that results will be the least evolved form of the species of the female. --- Please become a fan of my cat. www.facebook.com/MalikTheCat