When humans finally managed to colonize the planet Venus, they discovered a gigantic system of artificial underground tunnels there. It is assumed that a highly advanced alien race known as Heechee has constructed those tunnels thousands of years ago. Eventually, an abandoned, yet fully functional Heechee spaceship is discovered. One of the explorers boards it, which leads humanity towards the discovery of a space station with many similar ships - all with coordinates of unknown planets already punched in. This station becomes known as the Gateway, being humanity's only connecting point to the rest of the universe.
Since traveling to the potentially dangerous destinations involves a great deal of risk, only some particularly courageous (or perhaps reckless) adventurers, called "prospectors", volunteered to go on these journeys. The protagonist of the game is one of those prospectors, having won the membership alongside a small sum of money and a one-way ticket to the Gateway. Eventually, he discovers an ancient device that contain a warning against a mysterious alien species known as Assassins - and the prospector is entrusted with the task of protecting the humans from that threat.
Gateway is a text adventure with graphics, based on the Heechee universe books by Frederik Pohl. It is identical in interface and general concept to Legend's other text-driven adventures (such as the Spellcasting series): the player may opt for text-only input as well as selecting verbs from a list. Clicking on objects or characters within the graphical representation of the location provides a text description. Puzzles are mostly inventory-based, though there are logic-related tasks as well. The player character explores various planets during the course of the journey. In a few cases, it is possible to die in the game.