I think there is some mistake in the question. I don't recall any reaction of geminal dihalide with Zn or Na that forms alkenes.
Geminal dihalides can react with NaNH2 to form alkyne.
But you can form diene from geminal dihalides. If you use a weak base, you can get a diene as a major product.
The direction of E2 elimination is determined by whether the mechanism is more similar to E1 or to E1CB. If it is similar to E1, then the thermodynamically more stable product dominates; if it is similar to E1CB, then the direction is determined by the more easily removed hydrogen atom. A strong base like NaNH2 drives the reaction towards the E1CB side, which gives the alkyne formed.