There are all sorts of ways in which the human body is more complex than simple calories-in-to-fat-gained machines.
People eat too much and exercise too little. Many people don’t have a clue how many calories they ingest.
Look at serving sizes in restaurants; even just the average size of a plate in a home.
I know many people (myself included); who have gotten a handle on what they are eating and seen weight come off.
Nothing fancy.
And you think that is a good argument for “GMOs magically make you fat”?