I can confirm 100% you will need a whole new tailgate. I went on a mission a while back for many months researching whether it was possible to fit a cup spoiler when I had the GT spoiler from factory fit.
I got all the way to the point where Renault sprayed my cup spoiler, pulled my GT one off and uncovered many lumps and holes and weird indents under the GT spoiler. It wasn't really possible to get the angle for the connection of the cup spoiler and I didn't want it looking like I had hammered parts of my roof in. Shame I didn't get a picture of it really.
With the indent you have. I had that on mine and though 'oooo this is the hole to put my light into, perfect' but it wasn't :nope:
Your only option is to buy a new tailgate and cup spoiler, then maybe sell your tailgate with GT spoiler to someone else.