gmerlin_imgdiff: 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 Debian possible default full paths are:

/usr/bin/gmerlin_imgdiff - from package: gmerlin

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

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