img2mercgrd: 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/img2mercgrd - 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/img2mercgrd - from package: GMT

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

Error in other languages:
img2mercgrd: bevel nie gevind nie
img2mercgrd: comandă negăsită
img2mercgrd: comando não encontrado
img2mercgrd: commande introuvable
img2mercgrd: command not found
img2mercgrd: käsku ei ole
img2mercgrd: không tìm thấy lệnh
img2mercgrd: komanda nerasta
img2mercgrd: Kommando nicht gefunden.
img2mercgrd: kommandot finns inte
img2mercgrd: komut yok
img2mercgrd: nie znaleziono polecenia
img2mercgrd: níor aimsíodh an t-ordú
img2mercgrd: no se encontró la orden
img2mercgrd: no s'ha trobat l'ordre
img2mercgrd: opdracht niet gevonden
img2mercgrd: parancs nem található
img2mercgrd: perintah tidak ditemukan
img2mercgrd: príkaz nenájdený
img2mercgrd: příkaz nenalezen
img2mercgrd: команда не найдена
img2mercgrd: командата не е открита
img2mercgrd: 命令找不到
img2mercgrd: コマンドが見つかりません