To authenticate through LDAP using GeneXus Access Manager (GAM), you need to use any GAM External Authentication Type, it can be GAM External Web Services Authentication Type or GAM Custom Authentication Type.
The external web service or the external program should dialog to the LDAP to check the validity of the credentials. Afterward, this information is passed to GAM, to go on with the flow of the application.
See HowTo: LDAP Authentication using GAM.