Ok, today is my install date for my 07 Jetta. Same interior and dash as your GTI. Do NOT get the metra dash kit. The kit will not fit properly. Look at this thread from VW Vortex: http://forums.vwvortex.com/zerothread?id=3145877
Get the Scosche dash kit, it fits perfect AND has the proper factory smooth texture, unlike the BestKits dash kit for the GTi which is rough.
The Scosche dash kit can be found here at Sonix: http://www.sonicelectronix.com/item_8186_Scosche+VW2317B+-+VW2317.html

Also the antenna adapter can be found here: http://www.sonicelectronix.com/item_4440_Scosche+VWA3+-+VWA3B.html
And most importantly, the wiring harness. As you may or may not know, our VWs do NOT have a switched 12v ACC line in the stock harness, nor does it have the orange Illumination wire that dims the head unit when the head lights are on. This means that those ordinary harnesses for VWs will not give us the 12v switched line or illumination. Also, to get the VSS wire for the GPS (And with the AVICs it is important to have this connected to get the most accurate positioning) you can not tap a VSS wire behind the gauge cluster in many 06s and all 07s. That means in order to get the most out of your AVIC, you gotta buy the Pioneer CA-R-PI.221 VW/Seat/Skoda Steering Wheel Remote Adaptor. I got mine in the mail yesterday from Incarexpress in the UK. This adaptor is not sold anywhere in the US right now so I had to import it. Check it out here: http://www.incarexpress.co.uk/view_product.php?partno=CARPI221
You can not use their online ordering to order internationally, so just email them at [email protected] . It costs about 54 pounds shipped. That's $109US. Totally worth it. This harness gives you speed pulse wire, 12V ACC, illumination, REVERSE, AND full steering wheel controls for Pioneer head units and VWs. Tell them what you want, give them your shipping info, you want it standard airmail, and make sure you open up a free Google checkout account in order to pay them. It took two weeks from emailing them to getting it to my door. The item was back ordered and did not ship out until last Wednesday. Their communication was EXCELLENT and it took four business days to get from england to New Jersey. Do yourself a favor and get this adapter. It will save you from having to cut and run wires to your headlight switch for illumination, your car's computer/tail lights for reverse signal, your cigarette lighter for 12v switched, and gives you the VSS signal which is quite important.
Basically, it's like this, if your not installing a subwoofer and you are getting this adapter, you will not have to splice into a single factory wire or rip apart the whole dash. You just take out the 7 torx20 screws, plug in the nav system, mount the gps and your done. 20 minute install tops. Well, the VW dealership just called and my car is ready to get picked up from oil change.
Hope this all helps, I'm going to be installing my D3, iPod, XM, and Bluetooth in about an hour and will have pics.