grdspotter: 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/grdspotter - 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/grdspotter - from package: GMT

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

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