Server To GUP Listen on which Port----------
GUP to Client Listen on whcih port----------
Client to GUP Listen on whcih port---------
Client and server are listen same port 2967.
http://www.symantec.com/business/support/index?page=content&id=TECH93813
GUP Managed How Many System--------------------
Total number of clients:
The current iteration of the GUP role can be configured to support up to 10,000 clients. Previous to SEP MR3, the GUP was only capable of supporting up to 100 clients or 1,000 clients with SEP MR4. To ensure that the GUP is capable of updating a large number of clients, you may need to configure the GUP to handle more than the default
Can we create Windows XP as an GUP or we required Server OS Platform ----------------
GUP is just a normal client with additional function. So there is no special system requirement.
Since you are using it for 100 to 225 clients so always use Server OS for GUP ( as XP has a limitation of 10 concurrent connection ).
Rest it will be just like a simple client which might use at the max 2-3 gig of extra space on the HDD.
https://www-secure.symantec.com/connect/forums/sep-11-group-update-provider-gup-hw-and-sw-system-requirements-0