Renderotica

Several months ago I bought Loki's CBT bundle, but since I can't afford photoshop I use G.I.M.P. I'm trying to make a comic featuring a bunch of my recent work. The tutorial pdf is very good as well... for photoshop. I have two problems though:

1) In the instructions is says to select an individual panel under the layers, right click, and select 'Edit contents'. The thing is, that doesn't happen in G.I.M.P. There is no edit contents choice, which would pop up the layer in a new window. So instead I copy the layer and open a new window to paste it in place. I then open the picture as a new layer on the new tab. There is no 'Use previous layer to create clipping mask' option that I can find. So instead I scale it, position it, hide it, and use the freehand tool to cut it. Then I go back to the original tab and paste selection in place. I'm just asking, is there a quicker workflow I can do? It takes me  5-6 minutes currently for a single panel. Is that normal?

2) I have gray boxes on the right sign of the first panel I can't get rid of. Why?

The reason I haven't contacted Loki is that his product is based in photoshop and I'm trying to use it in G.I.M.P.! Any help would be appreciated! I tried to upload an image but it won't let me!

Thanks! GIMP does use layers and layer masks, but I don't really understand the masks yet. I'll watch some tutorials and try to figure it out. There is a duplicate layer option, but it doesn't go to a new screen, or let me move it to one. My current workflow is as follows:

1) Load template
2) Select a single panel layer
3) Copy panel layer
4) Create new project with transparency
5) Paste into place
6) Open picture I'd like to use as new layer
7) Resize and move it over panel layer
8) Hide art layer, while keeping it selected, and use the freehand tool to cut it out around the panel layer
9) Cut selection (the artwork) and switch back to the full template project
10) Paste selection in place
11) Rinse and repeat for every panel

It just feels like a lot.