Sorry to revive an old thread...
Part # 5G0012115N absolutely does fit in the GTI.
While searching the internet about this part i kept seeing posts saying it wouldn't fit the GTI. Then i kept looking at my 2020 GTI and then at the pictures of the part and thinking it really looked like it would fit. Finally i gave in and ordered the part. To be certain this is the Euro Golf R toolkit foam insert, part # 5G0012115N. It fit. It shoves in snugly but it slides all the way back. There are three bumps on the foam and they fit perfectly into gooves on the bottom of the cargo tray. i didn't find that out till after i had removed those bumps as i didn't realize there were gooves in the tray.
Your cargo tray has to be in the upper position for the toolkit foam to fit, so if you have the OEM MuddyBuddy mat, you won't be able to use it as that mat requires the cargo tray to be in the lower position. WeatherTech now sells a version of that mat that works with the tray in the upper position though, you just can't get it branded MuddyBuddy through VW.
If i can figure out how to post a picture, i can do that.