create a parallax object

"I have the creative template and I need to work out how to create a parallax object that stays in the same position but changes opacity. Can I do this in the widget short code? If not where do I go about placing it in the backend?"

Parallax effects are not pure html effects. They require at least some css and js coding.
Sadly - this is not kind of change we support.
You would need to know how to create parallax effect first, and then hook into gk.scripts.js and add some js there.
Current "pseudo" parallax on the site is more in detecting element entering viewport and adding a class to it - and then using css3 transitions to create animation than typical parallax.