Did I do that? I almost forgot what that was all about. Sorry, I'm not doing skinning anymore and I haven't worked that way in a LONG time seemingly
But yes, Gamary is correct, you select the space between the panel lines.
I'm talking Photoshop here though, I'm sure Gimp will give you the same abilities but they may have different names.
We'll need someone who knows both Photoshop and Gimp to fill in the proper operation names for Gimp.