grdfft: 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/grdfft - 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/grdfft - from package: GMT

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

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