gap5: 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/gap5 - from package: staden
/usr/lib/staden/bin/gap5 - from package: staden

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

Error in other languages:
gap5: bevel nie gevind nie
gap5: comandă negăsită
gap5: comando não encontrado
gap5: commande introuvable
gap5: command not found
gap5: käsku ei ole
gap5: không tìm thấy lệnh
gap5: komanda nerasta
gap5: Kommando nicht gefunden.
gap5: kommandot finns inte
gap5: komut yok
gap5: nie znaleziono polecenia
gap5: níor aimsíodh an t-ordú
gap5: no se encontró la orden
gap5: no s'ha trobat l'ordre
gap5: opdracht niet gevonden
gap5: parancs nem található
gap5: perintah tidak ditemukan
gap5: príkaz nenájdený
gap5: příkaz nenalezen
gap5: команда не найдена
gap5: командата не е открита
gap5: 命令找不到
gap5: コマンドが見つかりません