I'm not exactly sure, some more details might be useful.  But lemme give it a try. 

First, DHCP, PXE, and TFTP are working it seems like.  Your client gets an IP.  It gets the PXE boot menu listing.  You select something (a Ghost client floppy disk image) off that list, and it transfers to your client PC correctly.  The transferred floppy image boots up to DOS (PC-DOS or MS-DOS, whichever you're using.)  It loads up a NIC driver.  It then starts running the Ghost.exe client.  You select the "Ghostcas" and "Unicast".  At the bottom of the Ghost window, it'll say Initializing IP (something like that, it flashes by a bit quickly for me.)  It then says "Local IP address: #####".  Ghost should also show you a little window that lets you enter in the Ghostcast Session Name.

So when you say "unable to contact the server", are you referring to this stage?  You enter a GCS session name, but the client can't find it?  At the bottom of the Ghost window it says "Attempting to contact <###>"" but it never succeeds, and gives you an error "Unable to establish contact with session <###>"?

As always, the first thing I'd check is the network on the client.  Did the DOS level NIC driver load correctly?  When booting up, hit the F8 key and it'll step through the boot process a piece at a time.  If you see an error msg, that'll be where it comes up.  If the NIC driver loads correctly, then go through the Ghost client until you get to the point about Initializing packet driver/IP address thing.  It should show you what the client IP is.  Goto another system and just ping that IP address.  It should respond.  At least that way, you know systems can reach the client (even if not the other way around.)  If it doesn't respond, then you know your client has no IP and you can start from there.

What NIC driver was used for the Ghost boot floppy?  Try the Universal/UNDI one.  I dunno the HP-7782, but on my HP dc7800, the NIC driver I used to create my Ghost boot floppy came from Intel.  The one from HP and the one from Symantec worked poorly or not at all.  So try downloading the latest DOS NIC driver for whatever is on the HP-7782 and recreate a boot-floppy using that.  To me, sounds like the problem lies in your boot floppy - since you booted, got PXE, got TFTP...up to that is fine.

Make sure your Windows Server has the port opened on its firewall for the GCS.  I have a Program Exception on my W2K3 server for Ghostcast Server at C:\Program Files\Symantec\Ghost\GhostSrv.exe.  I dunno what version of GSS you have, and I don't know if GCS changes depending on the GSS version, but you get the idea there.

Course, I dunno what 19901 is.  It might be something completely different, and I'm sure Symantec folks will jump in too.  If you have a Ghosterr.txt file, you might post that as well.  I can't decipher it (would love to learn how though!) but again, the Symantec folks can :)

Good luck,
PH