gustaf: 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/gustaf - from package: seqan-apps
/usr/lib/seqan/bin/gustaf - from package: seqan-apps

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

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