I think, Galleria js handle the show and hide of that div's so if you want always hide works with !important
Why not add another class with "display : none !important"? this way overrides the original and let you change dynamically with jquery with addClass, toggleClass and removeClass statements
.myHideClass{
display:none !important;
}
so... how do I put the class on the div by default?
<script>
// Load the classic theme
Galleria.loadTheme('galleria.classic.min.js');
// Initialize Galleria
Galleria.run('#galleria');
//the divs are already created so...
$('.galleria-layer').addClass("myHideClass")
</script>