downloads.gs

var SHEET_ID = "17lRx0wgx9f2JutGKKFfkHmbrkqgsy7nf6IDJL07w-Vs";
var SHEET_NAME = "downloads";

function doGet(event) {
  return handleResponse(event);
}

function doPost(event) {
  return handleResponse(event);
}

function handleResponse(event) {
  var lock = LockService.getPublicLock();
  lock.waitLock(30000);

  try {
    var result = {status: "ok"};
    var sheet = SpreadsheetApp
        .openById(SHEET_ID)
        .getSheetByName(SHEET_NAME);
    var headers = sheet.getRange(1, 1, 1, sheet.getLastColumn()).getValues()[0];
    var currentRowNumber = sheet.getLastRow() + 1;
    var currentRow = [];

    for (var i in headers) {
      var header = headers[i];
      if (header === "timestamp") {
        // automatically insert a new date into the first column
        currentRow.push(new Date());
      } else {
        currentRow.push(event.parameter[header] || "");
      }
    }

    sheet.getRange(currentRowNumber, 1, 1, currentRow.length).setValues([currentRow]);
  } catch (error) {
    result = {status: "error", error: error};
  } finally {
    lock.releaseLock();
  }

  return ContentService
    .createTextOutput(JSON.stringify(result))
    .setMimeType(ContentService.MimeType.JSON);
}

1. Create a new Google Spreadsheet. It may be required to use a normal (non-company) account - I wasn't able to create a publicly available endpoint when the spreadsheet was on …@jah.pl, it worked flawlessly on …@gmail.com.
2. Adjust the spreadsheet ID in line #1 (when you open the spreadsheet, copy the "1bK3jI9TL5T8Tf0CmlVHpIlhg6Mu2-qJXDCmmip7rzGA" part from the URL - https://docs.google.com/spreadsheets/d/1bK3jI9TL5T8Tf0CmlVHpIlhg6Mu2-qJXDCmmip7rzGA/edit).
3. Adjust the sheet name to e.g. "downloads" (line #2).
4. Add following colums (the first row):
   timestamp | firstname | lastname | email | subscribe | country | organization | sector | explanation
5. Go to Tools -> Script Editor.
6. Paste the above script there.
7. Go to File -> Manage versions, save a new version.
8. Go to Publish -> Deploy as a web application, ensure to change permissions to anonymous access.
9. Adjust the endpoint URL in the code with the generated address.