Why do I need more gelatin in a half fat cream cheese cheesecake?
What comes to mind is the emulsifying properties of eggs. When eggs emulsify with the fat it creates a smooth or luxurious texture. Less fat might make the texture less desirable and the gelatin might improve the texture.

As for how well the cake will set, I don't think that adding more gelatin because there will be less fat is going to matter. Eggs are usually the setting agent in cheesecakes. 

Hope that helps!