gdm-screenshot: 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/gdm-screenshot - from package: gdm

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/gdm-screenshot - from package: gdm3

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

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