TinyDB (store/save coordinate results on the device)
You can not with Blocks monitor continuously without a StayAwake routine or an extension. App Inventor apps do not run in the background natively. A partial solution for your app using an extension:
Another way: use http://geojson.io/ to build your polygon; import the polygon as a FeatureCollection.
and another way
Search the forum for many more examples for pieces of your proposed app.
Here are some resources to help you learn to use the AI2 tools. A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook http://www.appinventor.org/book2 ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.