How was the guided daily amount (GDA) calculated?
The total calories are not in fact a recommended value - they are an example amount that is derived from some calorie intake that seemed reasonable to someone (I don't know the details). It does not mean that the government is recommending that you eat exactly 2000 calories per day.

The relative content is, however, a recommended value, based on presumably research. So, if you do take 2000 calories a day, then you know how much of it should be fat/carbs/proteins.

I think that there is some controversy about whether the recommended values were influenced by industry lobbying. Moreover, science is definitely not clear on what the optimal food content for a human is, which is why we keep having the different diet fads, oscillating from no-fat to no-carbs.