Hi Steve,
You are right in the part of Gateway Accounts, a easy way to explain is that is used to allow the SPA3102 to use more than one SIP proxy for the outgoing calls, accordling with your rules.
The VoIP Users is used to allow other ATAs to call through the SPA3102 (From SIP to PSTN), if you have for example 3 more ATAs and don't have a SIP proxy you can authorize them in the VoIP Users session.
The subscriber Info is the default authentication method in to the ITSP or IPBX.
Regards.
Andrey Cassemiro.