Gintama was the first manga to coin this term although it was later used by others to depict this nuance found in the Doraemon world. Originally, it was called "ジャイアン映画版の原理" or loosely translated as "The Concept of Cinematic Version of Gian".
Movie versions of Doraemon will always feature all 4 kids (Nobita, Shizuka, Gian and Suneo) together with Doraemon himself although others may make guest appearances (i.e. Doraemi). Due to the fact that the movie versions always come with its own antagonist, Gian (and his "sidekick" Suneo) will, at some point during the story, drop their antagonism and become friends of Nobita. The resulting effect of this oft-used plot device also sees Gian sacrificing himself as some sort of cannon fodder in the face of on-coming enemies so his comrades may escape and fulfill their missions.
And suddenly, Gian doesn't look so bad any more. His past crimes are forgiven. Everybody thinks he is a good boy at heart with his friends being his top priority. Until people watch next week's episode on TV and he goes back to tormenting his "friends" but by then, most people already have a different opinion of him. Gian is capable of good! He just needs the right situation to draw it out!
I guess you can see where this is going now and how "Gian Effect" can be used to describe situations in life - if you had been a bad person all your life and you do 1 good deed, people will think there is some redeeming values in you. However, if you had done good all the time but losing sight for a single moment in a lapse of judgement, and you may be branded a treacherous person, a snake and someone who hides a dagger behind smiling faces. Think Thor vs Loki at the animal shelter adopting a dog - who will win in terms of charm factor?
Have you ever encountered such a situation in life? I had probably seen more than my fair share, especially after I started work. These days, you might even be photographed and your "deed" plastered online and all over the papers - the years of good and honest living destroyed by a single moment of misunderstanding.
Ok, my train of thought is getting a little too depressing. Anyway, some of you probably didn't know that Doraemon is French and that Nobita is still being tormented by Gian even after they had reached adulthood. Here's proof. ;)