Open image gallery on link click

As I understand it, you want to show a div element containing the gallery when Show Gallery is pressed – otherwise to hidden.

This can be accomplised with a couple of jQuery lines:

$( ".show" ).click(function() {
  $( ".gallery" ).addClass( "display" );
});

With the appropriate CSS. See jsfiddle
Be sure to remember to include jquery in your theme if it doesn’t contain it otherwise.