Anyone ever figured out how to bring in data from one of the Google Analytics APIs? I'd like to create aggregated reports across all GA accounts I have access to.

You can do this in the Google Sheets using the Google Analytics Add-on but it's clunky at best for trying to get aggregated data together as it separates out each account onto a different sheet/table. Analytics Canvas (analyticscanvas.com) is another option but I'd really rather work with the raw data right in FileMaker.

I haven't used FileMaker in years so I'm not sure how easy or difficult it is to connect it to external APIs like the ones from Google so any help in understanding whether this is reasonable feasible and how to go about it would be much appreciated.

Thanks!

Since FM16, FM is really good at integrating with REST APIs (or anything you can touch with cURL really).

The "insert from URL" supports a great many of the cURL options, plus there are native Json functions.  If you are somewhat familiar with REST APIs you'll enjoy it.