My first thought on this was to obviously change the colour which I did in OnTriggerEnter() but found it was a bit of a pain to work out how to change the emission color.
void OnTriggerEnter(Collider other)
{
leftGate.GetComponent().material.color = Color.gray;
leftGate.GetComponent().material.SetColor("_EmissionColor", Color.gray);
rightGate.GetComponent().material.color = Color.gray;
rightGate.GetComponent().material.SetColor("_EmissionColor", Color.gray);
}