If you are using the single GUP option in RU5 or the GUP in the earlier version,the GUP functionality will work with respect o the group which it is located.This is irrespective of the subnets which clients present.(i.e,The GUP and the clients should be present in same group.Subnet will not come to picture at all.) .
If you are using Multiple GUP options in RU5,it is fully depend on the subnet which the clients present.It is not group dependent.i.e,Client will receive updates only from a GUP which is present in it's on subnet only until and unless a PC which not present in that subnet specified in the liveupdate policy (specify the host name or IP address of a Group Update provider on a different subnet to be used ,if Group Update providers in the same subnet is not available option.) .Also assure that you are assigning same policy for both the groups--for the group which client present and for the group which GUP is present.