I am also doing this soon on my 5 door MK5 GTI.
The research ive done so far.
Part NO: 1K6839407F041 bright black (unique to the GTI
Glossy) seems the rest of the MK5s are matt black. £43 new.
You need to remove the outer door skin. Possibly the glass im unsure.
Pretty much all the GTIs I've messaged about that are being broken for parts are similar. So best to get new part. If you paint it... you will need to do the other door to match.