Danny,
Most panel mounted TSO'd GPS's like the GTN/GNS series have digital ARINC 429 interfaces for interconnecting themselves to other devices. Saves a ton of wiring and offers better performance over the analog interfaces.
This module just provides the physical interface needed to marry the 430 to the G3X system digitally.
Prior to the new modular products, the ARINC 429 interface was built into the GSU73.