I have changed the way I am reporting the value of my precious metals portfolio. Now my portfolio and GDX are starting from the same point (December 16, 2015). As a result, it is possible to visually compare the price action of both instruments. Please, have a look:
As the chart shows, since December 16, 2015 the portfolio has increased its value from 9,997.1 to 10,255.6 (2.6% up), while GDX has fallen 6.6%.