I think you are confused. It is logical, because the two authors you mentioned have lived at México too much years. but they aren´t mexicans.
Gabriel García Marquez is from Colombia.
Isabel Allende is from Chile
The author of the book Like wather for chocolate is Laura Esquivel a mexican woman, but in my opinion not a very good writer.
I mentioned some mexican author:
Carlos Fuentes.- Novels and esays
Jose Revueltas.- Novels, teather, stories for movie (excellent)
Juan Rulfo.- stories
Rosario Castellanos.- Stories and novel (Balun Canan is the name of the must important)
Agustín Yañes.- Novels
Pita Amor.- Poetry
Octavio Paz.- Esays, poetry
Juan García Ponce.- Novels
Ines Arredondo.- storie
Jose Agustin.- Stories
Parmenides García Saldaña.- Stories
ant too much more.