gdf_merger: 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/gdf_merger - from package: gdf-tools

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

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