I've stupidly left the gas hob on for a few hours today?

No it will have been quite harmless so don't worry. Remember some recipes (eg boiling a xmas pudding) need a pan to simmer on the gas for that length of time! Restaurant kitchens have their gas jets burning for most of the day with no ill effects. Central heating burners are alight most of the day in winter and you wouldn't worry about that. When the gas burns it turns into water and carbon dioxide, which is what you breathe out yourself!