I have a Golang app running on Google Cloud App Engine that I can update manually with "gcloud app deploy" but I cannot figure out how to schedule automatic redeployments. I'm assuming I have to use cron.yaml, but then I'm confused about what url to use. Basically it's just a web app with one main index.html page with changing content, and I would like to schedule automatic redeployments... how do I have to go about that?
If you want to automatically re-deploy your app when the code changes, you need what's called CI/CD (Continuous integration/deployment). What a CI does is, for each new commit to your repository, check out the new code and run a test script. If all the tests pass (or if you don't have any tests at all), the CI server can then deploy your code to App Engine, all automatically.
One free (for open-source projects) CI provider is Travis CI. To configure it, you need to make an account with Travis, and a file called .travis.yml in the root of your repository. To set up App Engine deploys, you can follow this guide to set up a service account and add the encrypted file to your repo. It will run a gcloud app deploy from a container on their servers, whenever you push code to a certain branch (master by default) in your repo.
Another option, which avoids setting up CI at all, is to simply change your app to generate the dynamic parts of the page when it gets requested. Reading the documentation for html/template would point you in the right direction.