Let’s say you want to use Google Sheets as a simple database to store your incoming inquiries with all details people fill in the form submission. You can’t use Google Analytics for it as it doesn’t allow personal identifiable information like email address to be stored. That’s why we’ll pass the data through Google App scripts to a Google Sheet, for free.
You can do the same with Zapier, where the coding part is hidden behind their beautiful interface, but their free version supports up to 100 tasks / month.
Let’s start by creating a simple sheet – my contact form has several fields I want to capture, like name, email, and subject. And in addition I’ll record a timestamp in GMT. So I’ll have these 4 as column titles:
Now we need the app script to do the magic. We go to Tools – Script Editor… and a new tab will open to write your script. We’ll remove the placeholder with the code below, courtesy of Martin Hawskey. It’s slightly updated, as his original version wasn’t working for me.
// Usage
// 1. Enter sheet name where data is to be written below
// 1. Enter sheet name and key where data is to be written below
var SHEET_NAME = "Sheet1";
var SHEET_KEY = "insert-sheet-ID-here";
// 2. Run > setup
//
// 3. Publish > Deploy as web app
// - enter Project Version name and click 'Save New Version'
// - set security level and enable service (most likely execute as 'me' and access 'anyone, even anonymously)
//
// 4. Copy the 'Current web app URL' and post this in your form/script action
//
// 5. Insert column names on your destination sheet matching the parameter names of the data you are passing in (exactly matching case)
var SCRIPT_PROP = PropertiesService.getScriptProperties(); // new property service
// If you don't want to expose either GET or POST methods you can comment out the appropriate function
function doGet(e){
return handleResponse(e);
}
function doPost(e){
return handleResponse(e);
}
function handleResponse(e) {
var lock = LockService.getPublicLock();
lock.waitLock(30000); // wait 30 seconds before conceding defeat.
try {
// next set where we write the data - you could write to multiple/alternate destinations
var doc = SpreadsheetApp.openById(SHEET_KEY);
var sheet = doc.getSheetByName(SHEET_NAME);
// we'll assume header is in row 1 but you can override with header_row in GET/POST data
var headRow = e.parameter.header_row || 1;
var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
var nextRow = sheet.getLastRow()+1; // get next row
var row = [];
// loop through the header columns
for (i in headers){
if (headers[i] == "Timestamp"){ // special case if you include a 'Timestamp' column
row.push(new Date());
} else { // else use header name to get data
row.push(e.parameter[headers[i]]);
}
}
// more efficient to set values as [][] array than individually
sheet.getRange(nextRow, 1, 1, row.length).setValues([row]);
// return json success results
return ContentService
.createTextOutput(JSON.stringify({"result":"success", "row": nextRow}))
.setMimeType(ContentService.MimeType.JSON);
} catch(e){
// if error return this
return ContentService
.createTextOutput(JSON.stringify({"result":"error", "error": e}))
.setMimeType(ContentService.MimeType.JSON);
} finally { //release lock
lock.releaseLock();
}
}
We need to specify our Sheet so we copy the part between /d/ and /edit and paste it in the insert-sheet-ID-here quotes on line 5.
We can name our project however we like and save our script. As a next step, we select Publish – Deploy as web app, change the Who has access to the app setting to Anyone, even anonymous, and hit Deploy button.
The first time it will ask for authorization, click Continue and Allow and you’ll be presented with the web app URL:
Copy it as we’ll need it to complete our setup.
Now, let’s create the variables in GTM that will pass the data we want. In my case I have 3 text fields and I’ll use their class names coming from Contact Form 7 WordPress plugin.
This is what my button looks like:
<input class="wpcf7-form-control wpcf7-text wpcf7-validates-as-required" name="your-name" size="40" type="text" value="" />
Name custom JS variable:
function() {
var inputField = document.getElementsByClassName("wpcf7-form-control wpcf7-text wpcf7-validates-as-required")[0];
return inputField.value || "";
}
Email custom JS variable:
function() {
var inputField = document.getElementsByClassName("wpcf7-form-control wpcf7-text wpcf7-email wpcf7-validates-as-required wpcf7-validates-as-email")[0];
return inputField.value || "";
}
Subject custom JS variable:
function() {
var inputField = document.getElementsByClassName("wpcf7-form-control wpcf7-text")[0];
return inputField.value || "";
}
You can quickly test them in the Console of Chrome Developer tools: 
Timestamp custom JS variable:
function(){
var today = new Date();
var str = today.toUTCString();
return str;
}
Now we have all our variables and we’ll contruct our GET call:
https://script.google.com/macros/s/<replace with your web app URL>/exec?Name={{Name}}&Email={{Email}}&Subject={{Subject}}&Date={{Timestamp}}
Be careful to use exactly the same words as it’s case sensitive.
You can test how this works if you open the link in a new tab – you should get a success result {“result”:”success”,”row”:2} and if you check your sheet, it’s automatically populated:
We can remove the dummy data later.
Now let’s create the tag that will fire this in GTM, I’ll use a Custom Image tag. Give it a name, paste your string in the Image URL field and select your trigger (the form submission in my case):
Create the tag, enter Preview and Debug mode and let’s see what happens. Go to your form, make a submission and we’ll see our tag being fired:
Also, when we check our sheet, it’s already there: 
Like magic! 🙂
