Kafka's theme in "Before the Law" is that the pursuit of meaning is often endlessly difficult and, in the spirit of existentialism, it is an individual struggle. Existentialism means "existence precedes essence." You exist and then develop your essence, what is essentially you: beliefs, desires, wants, fears, etc. You are not predetermined to "be" anything. You make yourself. Thus, you are responsible for making yourself as it is an individual quest. With that responsibility can come anxiety because you are alone in that struggle.
There is an allusion to the gates of heaven here, but Kafka is speaking of granting entry to meaning, knowledge and truth. Kafka also sees bureaucracy as mimicking the existential quest for truth. The "law" represents meaning but also, analogously, the man-made systems (law, government, politics, academia, the aristocracy or owning class, etc.) which claim to create, sustain and categorize things like truth and knowledge. These systems also tend to keep others out. Often, being granted entrance into these groups is as difficult as discovering meaning; especially in an existential outlook, which means it is an individually responsible and usually uphill battle. It is up to him/her to go through the open gate.
The forlorn nature of the man and the doorkeeper is very existentialist in Kafka's work. The idea of this country man questioning the nature of structural integrity in the world is also highly existentialist. There is a forlornness indicating how alone human beings are when the man approaches the doorkeeper, waits there for years, recognizing that there is no oen else present. At the same time, the fact that the man does not passively sit, but rather raises questions and seeks to do bring meaning into his quest is another existentialist element. The individual differentiation offered by the gatekeeper in suggesting that this specific path was the country man's helps to bring to light the idea that individuals are alone in their search for meaning within consciousness.