I had my mk6 GTD for nearly 5 years before I sold it. It was pretty much faultless, and probably the best car I've owned.
As the GTD you're looking at has got a DSG gearbox, check that the gearbox service was carried out at 40k miles. Not certain at what age / mileage the cambelt should have been changed on an '09 GTD - think it's either 4 or 5 years, so it should have been done on a 6-7 year old GTD.
Check the diamond cut alloys for for kerb or whiteworm corrosion damage; diamond cut alloys are quite prone to whiteworm if the lacquer is damaged and water gets under the lacquer surface. It'll cost £80-£90 plus VAT per wheel to get a decent refurbishment done. Also check the rear tyres for sawtooth wear on the inside edge; if present (you should be able to feel the 'stepped' pattern to the tread if you run your hand around the inside edge of the tyre), this could indicate an alignment problem.
If it's got leather trim, this can get a bit 'saggy', especially on the right hand bolster on the driver's seat. If it's got cloth trim, check the right hand bolster on the driver's seat backrest for wear.
You've not said what the spec of the car is, but there are some options that are worth having;
- leather, but see above re. possible sagginess.
- xenon headlamps; these were an expensive option and not many GTD's had them but they are very good.
- RNS510 or RCD510 headunits. Both of these are touchscreen (first one has sat nav, the second has a 6 disc CD changer).
- Bluetooth, this is a fairly easy retro-fit if it hasn't got it.
- Dynaudio sound system.
Good luck; hope the car you're interested in is a good one, but remember, there are plenty more out there if it doesn't meet your expectations.