Dahlias should be lifted, the top growth cut off and left stem side down in trays of sand to dry out for a week or so, then potted into half sand half peat or compost, kept somewhere frost free, then repotted into proper compost next spring to start them into growth. Do not replant outside before end of May next year.
Gerberas don't survive here in London over the winter, no idea whether they will up there - they do survive in the West Country.You could try potting them up and removing to a greenhouse for the winter to give them some protection.