I am trying to select the darker areas by making a selection of marching ants in the tunnels of this picture. I want to draw a gradient across the tunnels brighting them up. I think I use the gradient tool but I don't know the workflow to make one side brighter and the other the other side slightly brighter.
I am not sure this is the type of result you had in mind. This this is how how it was done
In the Channels panel a dupe was made of the Blue channel (which will eventually become the mask), its contrast increased and the black and white a areas were touched up with either black or white in overlay modo. The result was inverted with this result:
In Layers a blank layer was added and the mask was applied to it. Then a selection based on the mask was made of the first archway and black?white gradient applied. This was repeated for each arch. The Blending mode was set to Linear dodge and opacity of the layer reduced to about 10%.