It's a 2004 or 2005 GT aggressor 1. I have looked but can not find the answer.
Which derailleur do I need?
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I'd start by measuring your wallet/bank account and see how much you want to spend on this. If you have nothing more than a bare frame, it could be expensive, more than a complete new bike.
Then measure the OLD of your frame, that's the gap between the inside of the rear dropouts. Its probably 130mm or 135mm. That will tell you what width of rear wheel hub you need.
From that the hub will limit the cassette width. If you can get a 8/9/10 speed rear wheel hub to fit, then you're well-positioned. If you can only get a 6 or 7 speed rear hub, then you're well into the "obsolete parts" I strongly doubt you'll get an 11/12 speed wheel hub in this frame.
Once you know how many gears you can get in the rear wheel, then that helps select a derailleur. You'll also need to have chosen front chainring/s too, to calculate the difference in chain links required, to know if you need a short/medium/long derailleur. Though its likely a MTB would have a wide cassette no matter if its 1/2/3 chainrings.
Lastly comes the groupset - you can get the same specs in cheap Tourney, or midrange Deore, or expensive XT/XTR. That comes down to what you can afford, and how you intend on storing the bike. Don't spend too much on a bike that lives outside or in dodgy areas like outside the local shops.
A couple of useful places online for these types of questions are bikepedia.com and bicyclebluebook.com . For me today, the former isn't working, but the latter is: https://www.bicyclebluebook.com/value-guide/brand/685/?familyName=Aggressor It doesn't show any Agressors from model years 2003-2005, but shows some in earlier and later years. I'd take a look at those other years. Remember to click on the photo of the bike and click through the value questions to get to the big bike photo and specs page. That will tell you about the derailleur and most other specs.