gap: 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/gap - from package: gap-core
/usr/lib/x86_64-linux-gnu/gap/bin/gap - from package: gap-core
/var/lib/wims/public_html/bin/gap - from package: wims

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

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