gap4: 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/gap4 - from package: staden
/usr/lib/staden/bin/gap4 - from package: staden

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

Error in other languages:
gap4: bevel nie gevind nie
gap4: comandă negăsită
gap4: comando não encontrado
gap4: commande introuvable
gap4: command not found
gap4: käsku ei ole
gap4: không tìm thấy lệnh
gap4: komanda nerasta
gap4: Kommando nicht gefunden.
gap4: kommandot finns inte
gap4: komut yok
gap4: nie znaleziono polecenia
gap4: níor aimsíodh an t-ordú
gap4: no se encontró la orden
gap4: no s'ha trobat l'ordre
gap4: opdracht niet gevonden
gap4: parancs nem található
gap4: perintah tidak ditemukan
gap4: príkaz nenájdený
gap4: příkaz nenalezen
gap4: команда не найдена
gap4: командата не е открита
gap4: 命令找不到
gap4: コマンドが見つかりません