I did not find any additonal information regarding Oliver Gib Jackson. Do you have any relatives still left in Brazos Co or any where else in TX? There are other sources in different parts of the state that I can refer them to if they are intersted in helping you with this part of your research. (I think I read that you were in Africa). This is the additional information that I had that may or may not be helpful:
There was an Elmore Williams who died in Harris Co 3/26/1911, Cert. #6033. Daisy Williams died in Waller Co 10-?-1918, cert #46311 and a Rosella Williams who died in Harris Co 8/17/1916, cert #19143.
Additional information about Boonville from th Brazos Co TX Cemetary Inscription book stated that Boonville was established in 1841 as the county seat of Navasota Co. The name of the county was changed to Brazos Co in 1842. Boonville flourished until 1866 when Bryan was established on the railroad. Residents moved to Bryan and the surrounding areas. Other Jacksons listed in the cemetary were Arthur(11/22/1895 to 1/23/1953)and was a Pfc 301 S. R, QMC; Clint Jackson (2/8/1881 to 5/18/1942); Lula Jackson (9/22/1882 to 6/21/1914).
In the Boonville Cemetary Book by Margaret Lips van Bavel it stated, "buried in the Boonville Cemetary are many slaves and their descendents". Many of the graves are unmarked.
I looked for information on that last name Syley and could not find anything. There was a family named Siler on the 1870 census (Campbell and Amanda) who had an 11 year old daugher named Mary. I know it said she was 18 when she married Gib Jackson in 1875. Other children in the Siler household were Julia age 17, Westley age 15, A 14 year old (I could not make out the name), Catherine age 2 and James who was less than a year old. Amanda's maiden name was Smithers. I found some very good info about the Silers! Many times names were misspelled and you have to look at other variations. Sometime ages are not exact. They were listed as Syler, Siler and Seiler. Campbell (also listed as Camel) was a preacher and farmer. In 1870 they are in Walker Co (Huntsville) and later appear in Brazos Co. He even has information naming slave owners back to NC and GA.
On the 1910 census a grandchild with the last name of Siler is listed (the head does not have that same last name), the Gib Jackson family is listed in the next household.