All true, The Gray Prince is the first novel in which the term Gaean Reach is used explicitly (indeed, in the first sentence of the Prologue), followed by Marune.

I believe that my memory was influenced adversely by the fact that the Hewett bibliography lists Trullion as "Gaean Reach #1". This is a logical deduction, since Trullion takes place in Alastor Cluster, the same as Marune and Wyst, in which the Gaean Reach is acknowleged. However, a logical deduction is not the same thing as the use of the term itself.

So now, you know you're a Wankher when you can cite the distinction between the questions: What was the first novel to mention the Gaean Reach?, and, What was the first Gaean Reach novel?