grdtrend: 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/grdtrend - 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/grdtrend - from package: GMT

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

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