Nice portrait and to be fair, luminosity masks are possibly more suited to landscapes but it is a tool that you can use.
It will all be in the Pat David blogs somewhere but on a basic level (the one I use) Too many screenshots to put in line and dialogues detached for clarity.
1. The masks in the channels dialogue can be used to make a selection. Just the same as you make a selection and save it to a channel for future use.
http://i.imgur.com/n4Lg4yK.jpg
Just remember after you use the channel dialogue, go back and enable a layer, otherwise you will be editing the channel
2. The more usual way. Create a layer mask using one of the channels. They are all there on a drop down.
http://i.imgur.com/Uqd0Y14.jpg
With visibility off on the lower layer you can see what remains.
http://i.imgur.com/A2S9tHa.jpg
Then all sorts of ways. Edit the layer mask or edit the layer under using any applicable tool. This one adjusting levels on the base layer.
http://i.imgur.com/AiUAAYy.jpg Edit: sloppy on my part, I should be adjusting the layer mask layer.
Since you have g'mic it is worth looking at the (interactive) color curves in the menu colors -> curves. That gives a choice of various color modes, this example using HSL curves.
http://i.imgur.com/63tgIYB.jpg