From page 2-7 of the G3X Installation Manual, revision AN:
When two external navigation sources are connected to the G3X system, the determination of which
source is #1 and which is #2 is determined by the ordering of the RS-232 connections to the GDU displays.
The #1 navigation source should be connected to a lower-numbered RS-232 port on the PFD1 GDU, and
the #2 navigation source should be connected either to a higher-numbered RS-232 port on the PFD1 GDU,
or to any RS-232 port on the MFD1 GDU. For additional information, refer to Section 2.5.1.
This sounds like the G3X is listening to the GNC 255 at startup. Check configuration on G3X, put GNX 375 on RS-232 #1 and GNC 255 on RS-232 #2, etc.