Gabino's death certificate does not name his mother and you probably already now. Since Gabino was a U.S. citizen he would have had a Social Security number. You can write to the SS Administration for a copy of the original application which he would have filled out himself. His parents names should be on the application.
I did find the death certificate for Silvestre Aguilar on Familysearch.orgYou can log in to see it for yourself and print it all free of charge.
Silbestre AguilarAge at death 58 years Pearsall, Frio, Texas Date of birthDec 31, 1897 written then crossed out with a notation--doubtful.Place of birth-Texas Date of death-- 08/30/1945Burial-Pearsall Date of death could actually be 1942 not 1945 Father -Nicanor Aguilar--born in Mexico Mother-Trinidad Casas --born in Mexico Informant--Gavino Aguilar--Pearsall.