gdiffmk: 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/gdiffmk - from package: groff

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

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