gacutil2: 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/gacutil2 - from package: mono-core

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

On Debian possible default full paths are:

/usr/bin/gacutil2 - from package: mono-2.0-gac

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

Error in other languages:
gacutil2: bevel nie gevind nie
gacutil2: comandă negăsită
gacutil2: comando não encontrado
gacutil2: commande introuvable
gacutil2: command not found
gacutil2: käsku ei ole
gacutil2: không tìm thấy lệnh
gacutil2: komanda nerasta
gacutil2: Kommando nicht gefunden.
gacutil2: kommandot finns inte
gacutil2: komut yok
gacutil2: nie znaleziono polecenia
gacutil2: níor aimsíodh an t-ordú
gacutil2: no se encontró la orden
gacutil2: no s'ha trobat l'ordre
gacutil2: opdracht niet gevonden
gacutil2: parancs nem található
gacutil2: perintah tidak ditemukan
gacutil2: príkaz nenájdený
gacutil2: příkaz nenalezen
gacutil2: команда не найдена
gacutil2: командата не е открита
gacutil2: 命令找不到
gacutil2: コマンドが見つかりません