Have you got the GFB DV+ T9356? If so I think that's only for Multiair engines rather than TJet. Some places list it differently showing only 124, Punto and US model 595 which have Multiair where as UK model 595s are Tjet.

Having vehicles with both engines Multiair in a Mito 170 and Tjet in a 595 Turismo,  I can say they have very different dump valve setups!

Both Multiair and TJet engined cars can have a Garrett turbo, but they have very different dump valve setups is my point.

The 124 setup you have seen a video for is the Multiair engine, the dump valve sits on the turbo and the GFB valve goes in between the factory dump valve and turbo. But in your TJet equipped 595 the dump valve isn't there, it's over by the battery, kind of in front and under the battery with pipe work going to and from it.

The GFB DV+ T9356 is for fitting directly on the turbo sandwiched between the factory valve, so if that's the valve you have bought, it's not going to do the job on a TJet engine.