grdfilter: 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/grdfilter - 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/grdfilter - from package: GMT

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

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