Modern gas hobs should be set up to turn out the gas if gas is flowing but there is no flame. My hob doesn't have this, but I think it obligatory nowadays. Apart from leaving the knob slightly open, it can happen that a low flame blows out from a draft or is extinguished by a boiling over. The modern cooker in my French chateau was dirt cheap, but has flame-loss protection.
If you can't afford a new hob, then you need to be careful how you twiddle your knobs, don't leave a nearby window ajar, and leave the lid slightly open if you are simmering stuff that might boild over (e.g. milk)