Which GPS doesn't do that?
Every GPS I've ever used does the same thing. Take a turn that isn't on the list, and within a few seconds it recalculates a new route for you based on that side street. My 2004-vintage TomTom GPS does that perfectly.
Excuse me! Maps was the first turn by turn I've ever used. sheesh. Before Maps, all I had was the static Google maps directions on my iPhone and it was a PITA to use while driving. Actually dangerous.