grdgradient: 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/grdgradient - from package: GMT

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

On Debian possible default full paths are:

/usr/lib/gmt/bin/grdgradient - from package: GMT

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

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