gac: 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/gac - from package: gap-dev

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

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