An online CW database lookup on George W. Martin (Co. K, 17th TX Cav.) does not indicate he was KIA in the WBTS. I suggest you order a copy of his military service record from NARA (National Archives & Records Administration; do a google search on NARA and you will find their website). Service record copie are "free" to ancestors.

Tom Simmons
http://www.civilwarvignettes.com