Each 'row' in your gallery is in fact an object with the fields that you set up in the data source you connected to that gallery.
Data Source: SP List called People
Fields: firstname, lastname, age, height, sex, etc...
The gallery shows each person as a row and that row, referenced by using 'ThisItem' inside that row/card, of the gallery will allow you to access that ojbects fields such as its name, age, height, etc. What you can do is utilize a collection or a variable to store that object so that it is accessable outside the gallery like this:
OnSelect Property of the item in the gallery set to one of the following...
Set(varPerson, ThisItem) -> varPerson.FirstName would return the name for that row/person.
or
ClearCollect(currPerson, ThisItem) -> First(currPerson).FirstName would return the same as above.
Once you have this you can build out a 'pop-up' screen as you mentioned that is outside the gallery and shows more info by using the objects fields that you just saved. Does this make any more sense?