x86_64-pc-linux-gnu-gcc-default64: 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/gap/bin/x86_64-pc-linux-gnu-gcc-default64 - from package: gap-dev
/usr/lib/gap/pkg/float/bin/x86_64-pc-linux-gnu-gcc-default64 - from package: gap-float
/usr/lib/gap/pkg/guava/bin/x86_64-pc-linux-gnu-gcc-default64 - from package: gap-guava-bin
/usr/lib/gap/pkg/io/bin/x86_64-pc-linux-gnu-gcc-default64 - from package: gap-io

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

Error in other languages:
x86_64-pc-linux-gnu-gcc-default64: bevel nie gevind nie
x86_64-pc-linux-gnu-gcc-default64: comandă negăsită
x86_64-pc-linux-gnu-gcc-default64: comando não encontrado
x86_64-pc-linux-gnu-gcc-default64: commande introuvable
x86_64-pc-linux-gnu-gcc-default64: command not found
x86_64-pc-linux-gnu-gcc-default64: käsku ei ole
x86_64-pc-linux-gnu-gcc-default64: không tìm thấy lệnh
x86_64-pc-linux-gnu-gcc-default64: komanda nerasta
x86_64-pc-linux-gnu-gcc-default64: Kommando nicht gefunden.
x86_64-pc-linux-gnu-gcc-default64: kommandot finns inte
x86_64-pc-linux-gnu-gcc-default64: komut yok
x86_64-pc-linux-gnu-gcc-default64: nie znaleziono polecenia
x86_64-pc-linux-gnu-gcc-default64: níor aimsíodh an t-ordú
x86_64-pc-linux-gnu-gcc-default64: no se encontró la orden
x86_64-pc-linux-gnu-gcc-default64: no s'ha trobat l'ordre
x86_64-pc-linux-gnu-gcc-default64: opdracht niet gevonden
x86_64-pc-linux-gnu-gcc-default64: parancs nem található
x86_64-pc-linux-gnu-gcc-default64: perintah tidak ditemukan
x86_64-pc-linux-gnu-gcc-default64: príkaz nenájdený
x86_64-pc-linux-gnu-gcc-default64: příkaz nenalezen
x86_64-pc-linux-gnu-gcc-default64: команда не найдена
x86_64-pc-linux-gnu-gcc-default64: командата не е открита
x86_64-pc-linux-gnu-gcc-default64: 命令找不到
x86_64-pc-linux-gnu-gcc-default64: コマンドが見つかりません