gkrellm: 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/gkrellm - from package: gkrellm

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

On Debian possible default full paths are:

/usr/bin/gkrellm - from package: gkrellm

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

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