gmcs: 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 CentOS possible default full paths are:

/usr/bin/gmcs - from package: mono-core

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/gmcs - from package: mono-mcs

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

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