As far as I know, the A3 and GTI are on the same "MQB" platform. That should mean that tuning and parts should be shared, for the most part. Both cars share the same engine and turbo, no worries
You'll notice that a lot of aftermarket parts are marketed for MQB platform cars so these parts should mix and match most of the time. Always ask the manufacturer for fitment confirmation if you aren't sure!
To see for yourself, perhaps browse ECStuning.com filtered for an A3 Quattro and look at performance parts. There is a fitment list provided and you should see the GTI and A3 together most of the time.
Just know that an AWD A3 is going to have some physical differences over the FWD GTI so you may need a different part number for your vehicle with some things like exhausts, I believe.
To make that shorter, they're the same in many ways and a tuned A3 should do just as well as a tuned GTI and you shouldn't have an issue finding parts for your car.
Hope this helps, post photos if you get the ride!
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