The API response is already JSON formatted so all you would need to do is pipe the output of multichain-cli to a file. But instead, you should probably just connect to the MultiChain API directly from whichever programming language you are using for your GUI, e.g. PHP, Java, Python, C#. Many third party libraries are available for doing this (see the bottom of http://www.multichain.com/developers/) or you can just use any standard JSON-RPC library.