GDP does not require that the goods or services were sold for money. GDP measures the market value adjusted to constant quality of all final goods and services produced. GDP can grow even with fewer workers, fewer hours, fewer buyers, and fewer units sold.

So the way I understood it, productivity, efficiency and quality gains can increase GDP year over year.

Say we are a country of 1 person. If that person can make a car in 2025, but in 2026 manages to make both a car and a house, the GDP has more than doubled. Doesn't matter that nobody paid the money for them.

Another weird thing is, say that 1 person country makes a car in 2025, and in 2026 makes a similarly priced car, if the car is higher quality, it counts as a higher GDP, because they'll measure its value as greater than last year's model, even if it sells for the same price, because the old model would now be worth less.