The algorithm behind GPT is based largely on accurately guessing for the next word given a sentence. This procedure is simple enough such that if you have a large amount of text data, you can write a simple script that can automatically retrieve the answer and you will get these solutions really fast with 100% accuracy.

This is also the reason why in some other industries, "training" procedure is much more cumbersome and expensive. Any field which requires experimental data (e.g. lifetime of a battery) is just not seeing as rapid progress with ML compared to other fields because there just isn't much experimental data and it is not easy to rapidly accumulate/conduct experiments. So training is difficult there in the sense that gathering big data is a huge challenge in itself.