If there is no analysis available, the user must supply in the meter configuration a suitable default relative density (= specific gravity), the density of the gas relative to air at the reference conditions.
If there is an analysis available (such as from a GC) then the AFC calculates the relative density (specific gravity) using AGA 8; any specific gravity coming from the GC is not used and not recorded.
The AFC has no ability to override the calculated SG with one entered from outside (e.g. from the GC), so any entered SG will not be used in calculations.
If one wants to record the SG generated by the GC, he can use a little-known feature called Arbitrary event-logged value (see Modbus Dictionary for registers 90-99), but be aware that there are only 1999 events and if the GC delivers a new analysis every (say) 2 minutes then this event will fill up the event log in less than 3 days.