The gateway machine generates new IDs for users that are newly registered. They may be placed on a UNC path that is accessible to the gateway process/service. So, specifying \\machine\ids\myidfile.id would put it on the network share.
There might be a need for the gateway to run as a user when configured as a service to get access to the share specified when a user is created. You can assign SYSTEM to have access to shares, but it depends on how the gateway network environment looks.
You can specify that the ID file be stored in the address book also by setting the Store ID In Addr Book resource attribute to TRUE/FALSE.