GSTest: 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/GSTest - from package: gnustep-examples

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

On Debian possible default full paths are:

/usr/bin/GSTest - from package: gnustep-examples

You may install the required package with command apt-get install {package}

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