run script only if checkbox is true

I have the increaseline1 and increaseline2 on buttons. I would have it so the button only works if a checkbox is set to True

function modifyCell(cellName, increase, checkbox) {
  var range = SpreadsheetApp.getActiveSpreadsheet().getRange(cellName)
  var toAdd =-1;
  if (increase) {
    toAdd = 1;
  }
   range.setValue(range.getValue() + toAdd);
 }

  function increaseline1() {
  modifyCell("Gus!G24!", true);


 function decreaseline1() {
  modifyCell("Gus!G24",false);