The skip forward and backward buttons work on gtcd but the keyboard
bindings "+" and "-" don't work.  I tried rebinding them to other keys with
no luck.  I was able to rebind the other actions successfully.

I glanced at the source code and it seems that the problem may be in gtcd.c
in fn skip_cb()  The function has conditionalals that check for mouse
presses and releases but it seems to ignore keystrokes.