In Gimp, how do I make part of the top layer transparent to

[Bert]

First off, is the top layer white, or any other solid color? Because in that case, you can just select that layer and erase the areas you want the bottom layer to show through on.

To make a fully transparent layer to begin with, select "Layer<New Layer". In the dialogue box that shows up, there should be an option asking which mode you want this new layer to be in; just click "Transparent", and voila. No erasing needed.