Why Gib is wearing an Australian Rugby League jersey (about 5mins 40s)
The Executive Producer of "The Sure Thing" was none other than Henry Winkler.
In the year prior to the movie's production, Henry visited Australia. My educated guess is that he was given or bought the jersey. I'd say John Cusack borrowed it for that scene or Henry Winkler asked him to wear it.
The jersey features the Australian Coat of Arms (with the kangaroo & the emu). It is worn by our national team when they play teams overseas or in Oz.
Cheers (from Australia), Jack.