In normal conditions and equal footings, Litwick has no hope of winning against a Gengar, no not one.
But footings aren't always equal so... here are some ways in which a Litwick defeats a Gengar:
- Level: If Litwick is way higher than Gengar in level, then it can defeat it.
- Gengar's remaining HP: If Gengar has really low HP, and Litwick has a Sash or something to take a blow, it can knock out Gengar in the next.
- Gengar's remaining moves: If all Gengar carries is Normal/ Fighting type moves, then it can't even touch the Ghost type Litwick. This pretty much gives the game to Litwick.
Note: Gengar is much faster than Litwick, so under no conditions (except the first one) will Litwick not have to take a blow from Gengar, which is usually the death blow. Hence most of the time Litwick stands no chance.
Hope I helped!