Hello Mark,
Google Tasks Import imports tasks to the Google Tasks server. If GTasks is synchronised with Google Tasks, then GTasks will show the imported tasks.
Note that I know nothing about GTasks, so I am unable to answer specific GTasks questions, but there is one possible issue with some third-party tasks applications in that they only access the default Google Tasks list. If that is the case with GTasks, then only tasks imported into the default task list will be displayed in GTasks. As I said though, I have never used GTasks, so this may not apply to GTasks.
I hope this helps,
Julie