Have you checked out this document? SPA ATA and Gateway Documentation
Since it has various links on the SPA3102 it may have the information you are looking for.
Cisco Small Business Support
On the spa3102, the ranges for PSTN to SPA Gain and SPA to PSTN Gain are -15 to 12, with a default of 0.
You may find the administrator's guide located here: http://www.cisco.com/en/US/docs/voice_ip_comm/csbpvga/ata/administration/guide/ATA_AG_v3_NC-WEB.pdf