gsix: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/bin/gsix - from package: gambit-c

You may install the required package with command yum install {package}

Error in other languages:
gsix: bevel nie gevind nie
gsix: comandă negăsită
gsix: comando não encontrado
gsix: commande introuvable
gsix: command not found
gsix: käsku ei ole
gsix: không tìm thấy lệnh
gsix: komanda nerasta
gsix: Kommando nicht gefunden.
gsix: kommandot finns inte
gsix: komut yok
gsix: nie znaleziono polecenia
gsix: níor aimsíodh an t-ordú
gsix: no se encontró la orden
gsix: no s'ha trobat l'ordre
gsix: opdracht niet gevonden
gsix: parancs nem található
gsix: perintah tidak ditemukan
gsix: príkaz nenájdený
gsix: příkaz nenalezen
gsix: команда не найдена
gsix: командата не е открита
gsix: 命令找不到
gsix: コマンドが見つかりません