Attitude to secular law

Rabbi, what is the attitude of the halakha to secular law? I saw that the Gamma of the Bible speaks of a person who testifies alone in a gentile court against his Jewish friend, putting him to death because he spends money based on one witness. What happens today with money that is spent not based on Torah law, or simply by the testimony of relatives or a woman (if we assume that she is disqualified from testifying today). What is our attitude towards this law? Does it have any value at all? Does it obligate us? And if so, is it only from the perspective of ‘Dina Demalchuta’? Is it permissible to testify alone in a court of law? Is a religious person permitted to be such a judge?
Thank you very much.