how to integrate the GTK# installer to my own application installer

According to this guide Deploy GTK Sharp applications to Windows without installing GTK# I decided to integrate the installer to my own installer I made using NSIS

I have a c# application which I wrote using gtk#. Now I need to install it to computers that don't have gtk# installed. I downloaded the installer of gtk# and I am executing it using:

ExecWait 'msiexec.exe /quiet /package "$INSTDIR\gtk-sharp-2.12.20.msi"'

That actually executes the installer and does something because I can see it in list of installed programs (the gtk package) but, when I try to launch the application after setup I get this error:

Unable to load DLL 'libgtk-win32-2.0-0.dll': The specified procedure could not be found. (Exception from HRESULT: 0x8007007F)

Is there any other package I need to install to make this work? Or what is a proper way to implement the installer?