gapi2-codegen: command not found

Have You tried accessing the binary with the full path?

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/bin/gapi2-codegen - from package: gtk-sharp2-gapi

You may install the required package with command apt-get install {package}
Error in other languages:
gapi2-codegen: bevel nie gevind nie
gapi2-codegen: comandă negăsită
gapi2-codegen: comando não encontrado
gapi2-codegen: commande introuvable
gapi2-codegen: command not found
gapi2-codegen: käsku ei ole
gapi2-codegen: không tìm thấy lệnh
gapi2-codegen: komanda nerasta
gapi2-codegen: Kommando nicht gefunden.
gapi2-codegen: kommandot finns inte
gapi2-codegen: komut yok
gapi2-codegen: nie znaleziono polecenia
gapi2-codegen: níor aimsíodh an t-ordú
gapi2-codegen: no se encontró la orden
gapi2-codegen: no s'ha trobat l'ordre
gapi2-codegen: opdracht niet gevonden
gapi2-codegen: parancs nem található
gapi2-codegen: perintah tidak ditemukan
gapi2-codegen: príkaz nenájdený
gapi2-codegen: příkaz nenalezen
gapi2-codegen: команда не найдена
gapi2-codegen: командата не е открита
gapi2-codegen: 命令找不到
gapi2-codegen: コマンドが見つかりません