Validate Gravity Forms fields

gf-validate.js

<script>
  
document.addEventListener("DOMContentLoaded", checkPage);

function checkPage() {  
  document.getElementById("gform_submit_button_2").style.display = 'none'; 
}

// first name
document.getElementById("input_2_1_3").addEventListener("change", checkFields);
// last name
document.getElementById("input_2_1_6").addEventListener("change", checkFields);
// company
document.getElementById("input_2_4").addEventListener("change", checkFields);

function checkFields() {
  var fn = document.getElementById("input_2_1_3").value;  
  var ln = document.getElementById("input_2_1_6").value;
  var fullName;
  var company = document.getElementById("input_2_4").value;
  var nameCheck;
  
  //console.log("fn:" + fn);
  //console.log("ln:" + ln);
  fullName = fn + " " + ln;
  //console.log("fulln:" + fullName);
  
  // if fields have content
  if (fn && ln && company) {
    // don't show if first = last
    if (fn == ln) {
      nameCheck = false;
    // don't show if full name = company
    } else if (fullName == company) {
      nameCheck = false;
    } else {
      nameCheck = true;
    }
  }
  
  if (nameCheck == true) {
    document.getElementById("gform_submit_button_2").style.display = 'block'; 
  } else {
    document.getElementById("gform_submit_button_2").style.display = 'none'; 
  }
  
}

</script>