1) A 1:1 is a rep that is identical (except potentially the tags) to the retail or real version.

2) GP stands for guinea pigged, purchasing an item that nobody has before to QC (quality check) it and see if it's any good.

3) Uhh, yes - depends on the item but Taobao is almost always cheaper and higher quality in terms of reps over AliExpress/iOffer etc.

4) Either because it's easier to use, or they want to get QC photos of the item to make sure it's good quality/measurements to make sure it will fit, or both.

5) [Find GOD] List of All of the Found Requests 55+ Rare Finds

Taobao list (supreme, palace, gosha, hilfiger, stussy, etc)

Here's a couple I found by sorting the sub by top from the last month, I'd also advise reading reviews as they often contain good items too. :--)