glxgears: 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/glxgears - from package: glx-utils

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

On Debian possible default full paths are:

/usr/bin/glxgears - from package: mesa-utils

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

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