It's not a real language, but he created a unique system with its own grammar, vocabulary, and alphabet. You can find examples of Gnommish throughout the books, particularly when Artemis Fowl communicates with the fairies.
Colfer even created a "Gnommish dictionary" in the book "Artemis Fowl and the Lost Colony," providing a helpful translation for readers. However, it's important to note that the Gnommish language is largely based on the author's imagination and isn't meant to be a fully-fledged, functional language.