grdmath: 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/grdmath - 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/grdmath - from package: GMT

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

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