iostream.o: 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/lib/x86_64-linux-gnu/gap/bin/iostream.o - from package: gap-dev

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

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