Use external switch to control blower fan on furnace
Connect the switch from R to G

Wiring a standard light switch between R and G will do what you want -- the switch will perform the exact same function as the fan switch on a thermostat in gas mode.  Namely, you'll be able to run the fan without heat by turning the light switch on, but the furnace will run the fan on its own when heat is called for, using its internal fan-thermostat.