gacutil: 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/gacutil - from package: mono-core

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

On Debian possible default full paths are:

/usr/bin/gacutil - from package: mono-gac

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

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