Howard Scott Gentry wrote his original booklet 'The Agaves of Baja California' in 1978 and it was published by The California Academy of Sciences. He incorporated the information from this work in his great work ' Agaves of Continental North America' in 1982. Since that was published a number of new species have been named and some revisions of his original studies have been undertaken.
Writing in Haseltonia in January 2015, Robert Webb and Greg Starr have made an important revision to the agaves of that peninsula and I enclose a link to the article below:
The same authors have studied specifically agave gigantensis and agave sobria, having travelled extensively through the habitat. It would appear that most of the plants labelled as agave gigantensis in collections and on nursery sales lists are in fact forms of agave sobria and that the true agave gigantensis has a very specific habitat near the summit of Cerro el Potrero in the northern interior of the Sierra de la Giganta. The plants are solitary, the leaves are green and not grey and the inflorescence is erect, unlike that of agave sobria. I enclose a link to the article below:
Baja California seems to be a fashionable topic at the moment , as John Pilbeam has just published (August 2015) a new book on the cacti and succulents of Baja California.