On December 15, 1847, 150 settlers petitioned the Texas legislature to establish a new county, which they suggested be named either "Pierdenales" or "Germania". The legislature formally marked the new county off from Bexar and Travis counties on February 23, 1848, named it after Capt. Robert A. Gillespie, a hero of the recent Mexican War, and made Fredericksburg the county seat. Gillespie County originally included areas that today are parts of Blanco, Burnet, Llano, and Mason counties. It underwent the first of five boundary changes in 1858, when the legislature formed Mason and Blanco counties, changed the Llano County boundary and established the present northern and eastern boundaries of Gillespie County. The last change came in 1883, when the county's boundaries were redefined and its present limits set.
-Handbook of Texas Online: Gillespie County