I looked at the TX death index at the Austin State Library from 1908 to 1940 and did not see a Gib Jackson listed. I did find a book in the Brazos Co. section entitled Brzaos County TX Cemetary Inscriptions. It stated, Gib Jackson born ? died ? wife named Mary.
Mary Jackson 11/15/1860 died 5/1/1908 wife of Gib. They were in the Boonville Cemetary. There was also a book on Brazos County Marriages listing a Gib Jackson age 20 who married Mary Syley age 16 on October 19, 1875. Joe Hurley officiated. There was other information I located in another book relating to births and Gib Jackson was listed as the father, including a set of twins born July 11, 1891. I did not know if you already had this information or not. Only birthdates were given, no names. The mother's name was not listed. I will be happy to give you the remainder if you don't already have them.