gmtdefaults: 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/gmtdefaults - 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/gmtdefaults - from package: GMT

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

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