look at the screen shot I posted the red circle is where you can change the "opacity" of the working layer and or the "mode" on the right side of image..the :mode" is a drop down arrow, which opens a window and gives you a multitude of choices to choose from, the "opacity" is a slider. this is your layer window, make sure you have the correct "layer" selected....
with just these two layers I don't think it much matters what is the bottom layer, you can see in the image I posted I have the map as the bottom layer, which means I copied and "pasted as new layer", the white image with all the cross marks onto the map.
if you want to make it one image......right click on the layer stack the top layer and merge down, that's how it works in GIMP always merge down from the top in the layer stack when merging layers.