How to safely connect a gas insert to flexible steel?
Is there another safe way to make a connection the this hose?

NO there is not a SAFE way to use the damaged line in your photo.
A gas supply line has fittings pressed onto to it that are engineered for Flammable, dangerous, gas under pressure.
You need a new supply line.

To replace this part would require disassembling the fireplace,
opening up the wall, removing the baseboard and making an access whole
in a recently installed hardwood floor.

WHY? There has to be access to the other end where the the line is connected to a shut off valve, NO?