gui-compare: 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/gui-compare - from package: mono-tools-gui

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

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