Normally it varies between 1/2 tsp to 1 tsp per pint (assuming fermented to dry) but if you're putting 1 gallon into a 5 gallon keg thats a lot of head space to fill and the oxygen in it is not going to do your brew any favours.
Have you considered the cheap lemonade bottles from ASDA or Tesco? Those might be more suited to a 1 gallon run.