I haven't, but took a quick look at the API docs. I think the code to add a row in a FormIt postHook would pretty simple (untested):
$Spreadsheet = new Spreadsheet();
$Spreadsheet->
setSpreadsheet("test spreadsheet")->
setWorksheet("Sheet1")->
add(array("name" => $hook->getValue('name'), "email" => $hook->getValue('email') ));
Setting up the Google Client with the access tokens, etc. looks more complicated. See the spreadsheet class here:
https://stackoverflow.com/questions/31748183/google-sheets-api-insert-a-row-with-php.
You might not need all the functions included there, but note that it's incomplete. It doesn't include loading the Google vendor autoload file or doing the authentication.
You can see some of that here:
https://developers.google.com/sheets/api/quickstart/php.
Hopefully, someone here has done this and can be more help.