@RoScha
Yes, the GalleryX.Selected will refer to the currently selected record in the Gallery. This may change based on actions within your app though. If you need to take a snapshot of the record at the time of selecting a record, then I would utilize a global variable to capture the currently selected item and not rely on the .Selected property of the gallery. This, again, will depend on your app.
As for selecting a specific record by default, then you can utilize the Default property of the Gallery to do this. Internally, PowerApps will maintain the current selection unless the underlying datasource is changed and it impacts the Items property significantly (i.e. there is more to your Items formula than just a datasource). In that scenario, you would need to provide logic in your Default property of the Gallery to lookup the record you want (and it must match EXACTLY to the record schema of the Items property).
The other thing you can utilize is the Select function. For Select, you can, for example, do Select(yourGallery, 3) this will select the 3rd item in your Gallery. The unfortunate part of that is that you really never know the item position of any of your gallery items to effectively select the right one...but in some cases you can surmise the correct row.
I hope this is helpful for you.
