I've just build gretl from sources  on a Ubuntu 16.04. When I try to install gig addon, gretl shows a error:
<html>
 <head>
  <title>404 Not Found</title>
 </head>
 <body>
  <h1>404 Not Found</h1>
  The resource could not be found.<br /
Why?, while the other packages run well.

Matteo Foglia