isogmt: 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/isogmt - 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/isogmt - from package: GMT

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

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