usually you can use the std gauge. just need to check the resistance values coming out of the gas senderr and compare with resistance coming from petrol sender. If the resistances are the same you should be able to easily adapt a manual change over switch to select either gas or petrol. If the resistances are different you may be able to change the gas sender resistance to match the petrol and still setup a change over switch.
I dont think I've got hold of it, so I hope this helps.