Please note - this is NOT a report about the gates being different heights, I already know that it's a feature.

When fence gates are stood normally they have smooth lighting (if enabled in video settings) but light on gates which are lowered to fit with a stone wall is not being smoothed.

I used a resource pack with less contrast in the wood textures in order to make the problem more visible, however it does occur when no resource packs are selected, the only difference is that normal gates just seem darker and the smoothing is not really visible. No custom block models were used, this is a flaw in the default model files which COULD be fixed with a custom file but would still be a problem with no resource pack on.

Also note that the fence gates in each screenshot are both of the same type (First 3 screenshots are oak, 4th is birch).