How can I compare two Google Docs documents within an Appian application?

In my opinion you will have to download a local copy of those docs in Appian. Then work with plugin Apryse WebViewer Component Plug-in . It features document comparison in semantic categories. Compiling the output from the plugin execution you can then show them in Appian. 

Apart from plugin, You can build AI Skill as well to compare contents of the documents and get the differences. But as AI can make misses, a review will be better. So you can try working with AI skill as well depending on how structured the docs are. 

For inline side-by-side comparison, you can use the Draftable plugin from Appian AppMarket and Apryse WebViewer Component Plug-in, but it is a third-party dependency and Appian does not officially support it, so debugging can be difficult.

If AI is enabled in your environment, I recommend using AI Skill + AI Agent together. Results depend on your prompt and input fields, but this approach is future-proof as Appian is actively enhancing AI capabilities - and document comparison being a common use case, better native support is expected soon.

Recommendation: Go with AI Skill + AI Agent for a more reliable and future-proof solution.