You can try this on your app.yaml assuming all your html files are in static folder.
handlers:
- url: /(.+)
mime_type: text/html
static_files: static/\1.html
upload: static/(.+)
Meaning it will match all and look up static folder with .html extension if you want specific files you can do
- url: /(hello|world)
mime_type: text/html
static_files: static/\1.html
upload: static/(.+)
means files hello.html and world.html only, to avoid handling all urls.