Well the Gerbe you posted is freehand so it will still be quite hard to sharpen with it. And it only has one grit you can go with. So I' d suggest getting the Spyderco sharpmaker which has two different grits and a preset angle, so that should be perfect for your Spyderco Harpy.
But yes it will work, but it will take some practice to get a decent edge, especially on the Fully Serrated Harpy.
He said "pocket" sharpener.
I suppose that one would work, but I'm in love with DMT products. They make some folding rod sharpeners, I believe.
Something like this:
http://yourcornerstore.com/index.php?main_page=index&cPath=4653_4672_4688