When use replicated volume with a user that has multiple group memberships, only the
primary group is honored when trying to write to a Gluster mounted
volume.  Here is an excerpt from my system:

[cdm@snake ~]$ groups
samba buh business buro antiq limit_a admin mnagrad projects

[cdm@snake 1clogins]$ ls -la
total 44
drwxrwxr-x 5 ubuhgalt buh  4096 Jan 18  2010 .
drwxrwxr-x 8 ubuhgalt buh 20480 Jan 20 16:45 ..
drwxrwxr-x 2 ubuhgalt buh  4096 Jan 18 19:49 admin
drwxrwxr-x 2 ubuhgalt buh  4096 Jan 20 16:45 buh
drwxrwxr-x 2 ubuhgalt buh  4096 Dec 27 18:09 pom

[cdm@snake 1clogins]$ mkdir admin/1
mkdir: cannot create directory `admin/1': Permission denied 
[cdm@snake 1clogins]$ touch admin/a
touch: cannot touch `admin/a': Permission denied

In the log I see it only looking up the user's primary group ID:
[2011-01-20 17:01:02.468859] W [fuse-bridge.c:184:fuse_entry_cbk] glusterfs-fuse: 10: MKDIR() /db1c/kb/1clogins/admin/1 => -1 (Permission denied)
[2011-01-20 17:01:02.469642] T [rpc-clnt.c:1295:rpc_clnt_record] : Auth Info: pid: 12067, uid: 500, gid: 111, owner: 47180423730016

[2011-01-20 17:04:13.96275] W [fuse-bridge.c:1514:fuse_create_cbk] glusterfs-fuse: 20: /db1c/kb/1clogins/admin/a => -1 (Permission denied)
[2011-01-20 17:04:13.96305] T [rpc-clnt.c:1295:rpc_clnt_record] : Auth Info: pid: 12121, uid: 500, gid: 111, owner: 47180423730016

fstab entry for gluster volume:
localhost:1c            /smb/1c                 glusterfs log-level=TRACE 0 0

snake# gluster volume info 1c

Volume Name: 1c
Type: Replicate
Status: Started
Number of Bricks: 2
Transport-type: tcp
Bricks:
Brick1: 1.1.10.5:/glvols/1c
Brick2: 1.1.10.6:/glvols/1c