gap.sh: 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.sh - from package: wims

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

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