Changing opacity level of the subtitle on GoGetSlider

It is impossible to do with theme options. You should add css rule in style.css at the bottom (via menu Appearance=>Editor):

1) if you use dark skin

.section-dark .header-content {
    color: rgba(221,163,75,1) !important;
}

2) if you use light skin

.section-light .header-content {
    color: rgba(221,163,75,1) !important;
}

You can change last value 1 from 0 to 1 for opacity.