I actually was having the same issue in my project. GetType is actually a method in C#'s base Object class.
So, just add this to your Metadata.xml to correct the bindings:
<attr path="/api/package[@name='com.spotify.sdk.android.authentication']/class[@name='AuthenticationResponse']/method[@name='getType']" name="managedName">GetResponseType</attr>
Seems to be working fine for me now. That should solve your issue.