Back in 1992 my friend Richard recommended a fantasy trilogy with a young man called Gill as the main character. Modern day boy gets transported to another fantasy world etc.. The ending of the book where he triumphs over evil, results in the real world (his world, where he is an unknown hero ) having the text on the U.S. Dollar bill changing from "in God we Trust" to "in Gill we Trust". I borrowed the 2/3 books from the local library, however cannot recall the name.
The Buffer Zone--that was what the seven kingdoms standing between mortal Earth and the realm of the immortal elves was called. When Gil Tyner planned to celebrate his 21st birthday with his parents, he didn't have any idea that he was about to spend not only his birthday but possibly the rest of his life in this zone.
Gil was just another college student when his father died and left him an incredible inheritance. Destined to be both the hero of Varay and the heir to its crown, Gil stepped through the magical doorway where he received a cram course in the hazards of the Varayan buffer zone. But now he would need all his skill, luck and wizard and warrior allies to face a new menace.
As the forces of apocalypse threaten the worlds of Earth, Fairy, and Varay, Gil Tyner must face the great Earth Mother Herself in battle if he is to prevent the end of the universe.
The last book in the trilogy was published in 1992, which fits the timeline.