After a longitudinal research on the traditional fruit varieties of his home Goriška region, Gregor published two books on the subject: An informative booklet "Stare sadne sorte na Goriškem" (Traditional fruit varieties of Goriska) and the monograph "Sadje sonca" (Fruit of the sun).

"Sadje sonca" is an illustrated collection of 64 found local fruit varieties, with scientific descriptions as well as written memories and tales connected with them. The book was illustrated and designed by Barbara Ogrič Markež and was conceived as a gift to the local fruit farmers.

An English translation and reprint of the book "Sadje sonca" is planned for the year 2023.