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Description of problem:

I have played around with /etc/group and /etc/gshadow to 
elucidate the behaviour of the same. 

First off, I noticed that group passwords are not handled
properly by 'newgrp', but that is another story ;-)
for which bugs 85280 and 14464 should be consulted.
The links to these:
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=14464
https://bugzilla.redhat.com/bugzilla/show_bug.cgi?id=85280

Here's the 'new' problem : The group membership list in /etc/gshadow
is not operational. Try this:

Create a file that is only accessible by group 'foo'
  
  ----rw----  1 nobody foo 18 May 30 19:33 /tmp/README
  
User dtonhofer is in group dtonhofer only
  
  That user cannot access the file, as expected

Add user dtonhofer to the membership list of group foo in /etc/group
then log in as that user (Access control is not dynamic, you have
to create a new process? Well, that's correct, probably..)
  
  User can access the file, as expected.

However, add user dtonhofer to the membership list of group foo in
/etc/shadow then log in as that user (I made sure the group password
was empty, too):

  User CANNOT access the file, but should be able to.

Now I know why the membership lists are in /etc/group instead of 
/etc/gshadow, as one would expect them to be. 

Hmmm....gshadow seems to be the unloved relative. Is it actually
ever used in the 'real world'? 

How reproducible:
Always