sampleGAS.gs

//Here's sample with doGet

function doGet(e) {
//e is an Javascript object check it in GAS Guide to know its parameters
  switch(e.parameter.functionName) {
    case "Hello":
        runHello()
        break;
    case "Cool":
        runCool()
        break;
    default:
  }
}

function helloF(){

//Your code block

//We need to do an HtmlOutput because it's an Web App
return HtmlService.createHtmlOutput("Hello Back !")
}

function coolF(){
return HtmlService.createHtmlOutput("Cool back !")
}