Lan,
I have always taken G.3.1.3.2 to mean the same FOSSIL fuel as the proposed design to prevent switching between natural gas, fuel oil, coal, and propane. Reading electric might feel okay until you get to the natural draft clause.

Assuming your Proposed electric heating consumption includes both your misc. heaters and VFR compressors operating in heating mode, I suggest your system is Fossil/Electric Hybrid heat and that you should model system 5 with natural gas boiler(s) and check the 'other' box noting hybrid heat. If you want to be compared against electric heated (system 6 with fan powered boxes & electric resistance), you need to get the gas in your OA unit set aside like an old CIR did for gas preheat.

The beta LEED interpretations database has elevated some CIRs to be precedent setting for LEED 2009+, maybe that gas preheat one made the list. Their website has been a bit skittish for me lately, so I suggest you to go USGBC.org, use the sign in link and then go to LEED - Certification Tools - LEED Interpretations. I say skittish because I get an error if I try navigate to the interpretations page and then log in.

Paul Riemer, PE, LEED AP