I can break it down.
Accessports are very similar but there are a few details to keep an eye out for. The Accessport you need for your car (MK7 GTI) is an AP3-VLK-002. The 003 is meant for the Golf R and S3.
Within the AP3-VLK-002 there are a few options. Just the AP (accessport) a new one will cost you $700 WITHOUT DSG tuning. You can buy a new one with DSG tuning for $1100. You can also buy an AP that wasn't meant for DSG and unlock DSG tuning for $400. DSG tuning is a software thing, it's simply them allowing the AP to communicate with the TCU. Mind you on top of all of this you will also need to purchase both the engine and transmission tunes from EQT. EQT charges $175 for a staged ECU (engine) tune and a separate $175 for a DSG (TCU) tune.
The TCU is the transmission control unit. Think of it as your transmission's ECU. The ECU tunes are transferable between any GTI from 2015-18. The changes in 19 onward mean it is a different file. The TCU tunes are locked to the TCU ID. This is where it gets tricky because there are multiple TCU ID out there and your ID needs to match the DSG tune file. There is a chance that you could get lucky and find an AP that has the staged tune you want and a TCU ID that matches your own. There is a more likely chance that you find a used AP with the staged tune you want and will need to just get a separate DSG tune.
In other words your options are:
New AP w/DSG + ECU tune + DSG Tune = 1,100 + 175 + 175 = 1,450 (assuming EQT doesn't do a bundle deal)
Used AP w/DSG that has staged engine tune & TCU ID matches = ~ 850 (based off what I've seen in the F/S section)
Used AP w/DSG that has staged engine tune & TCU ID doesn't match = ~ 850 + 175 = 1,025
Used Manual AP w staged engine tune + DSG unlock + TCU tune = ~ 500 + 400 + 175 = 1,075
and to
@GTI Jake point always make sure a used AP is unmarried otherwise it is a brick. Divorce is messy even with AP's.