The Gold upgrade is per app (so one project) and lasts for 1 year. This means that you can continue to make changes, add new features etc. to that app as many times as you wish within that year. After that, your app continues to work of course, but if you want to make additional changes to it, but still want the features of the Gold package, it will require another Gold upgrade.
Hope that helps.
Do I can update my app without having the gold package?
Or do I require the gold package