Can you file a civil lawsuit against your husbands mistress for value of gifts she received from your husband?

You could try, but chances are it isn't going to get anywhere. Gifts are just that, gifts. Though you knew nothing about them at the time, they were given by your husband. If your husband is deceased, the estate may be able to retrieve gifts given in the last couple of years. Consult an attorney in your area, but chances are there won't be a case.

Added: If you sue your husband for divorce, you would probably be able to get some sort of compensation for the gifts in the divorce settlement.