I assume this is your first time ever using that tool?
The data import is only for the items that exist in the game in general (this has to be done the 1st time after installation and everytime new items are introduced to the game), not for the items that you collected.
To add those into your GDstash, you have to select the “Stash Transfer” or “Char Transfer” and use one of these buttons
When I want to transfer items from my ingame stash to GDstash I put them all in one stash tab, then select that tab in GDstash and click Move entire page to DB.
When you need an item that you previously added to GDstash, first make sure to click the reload button, otherwise it may happen that you overwrite your ingame stash. Then just click the big search button in the upper left, select what to search for and when the search is done, transfer the item you want by selecting it and then clicking on an empty space in your game stash on the right, then click Save Stash.
It’s important to know that Stash Transfer and Char Transfer are 2 different stash files, so I’d recommend only using the Stash Transfer so everything you collect is in the same place.
I actually have no idea what the “Collection” tab does, I never used it and it’s not necessary for transferring items to and from the stash.
edit: it seems the Collection tab is just for keeping track of item types you added to your GDstash. So once an item of a certain type is added, it gets a green check mark. If none of that item type has been added, it gets a red X. Might be a useful feature for completionists who want to collect at least one item of each type.