For now (July 2019) the website gives you the ability to build a vocabulary the most used 5000 nouns in German language. You will be able to practice Der, Die, Das as you increase your own vocabulary. You can add new words into it and focus on the hardest words, if you prefer.
In the future, new options may be added to use the service as a full vocabulary tool. New types of users may be added as well.
DerDieWhat.com will be free until it is not. As an early bird, you get the opportunity to use it as you please. In a matter of months however there may be a very small fee applied to registered users, to cover hosting costs at least.
Until then, feel free to use this resource and keep practicing.