gmt2rgb: 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/gmt2rgb - 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/gmt2rgb - from package: GMT

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

Error in other languages:
gmt2rgb: bevel nie gevind nie
gmt2rgb: comandă negăsită
gmt2rgb: comando não encontrado
gmt2rgb: commande introuvable
gmt2rgb: command not found
gmt2rgb: käsku ei ole
gmt2rgb: không tìm thấy lệnh
gmt2rgb: komanda nerasta
gmt2rgb: Kommando nicht gefunden.
gmt2rgb: kommandot finns inte
gmt2rgb: komut yok
gmt2rgb: nie znaleziono polecenia
gmt2rgb: níor aimsíodh an t-ordú
gmt2rgb: no se encontró la orden
gmt2rgb: no s'ha trobat l'ordre
gmt2rgb: opdracht niet gevonden
gmt2rgb: parancs nem található
gmt2rgb: perintah tidak ditemukan
gmt2rgb: príkaz nenájdený
gmt2rgb: příkaz nenalezen
gmt2rgb: команда не найдена
gmt2rgb: командата не е открита
gmt2rgb: 命令找不到
gmt2rgb: コマンドが見つかりません