If the input data are in a separate sheet of the same Google document, then you can still use the same logic. The function just changes to reference a different sheet:
=IF(Sheet1!B2>Sheet1!A2, "⇧", "⇩")
I have updated the document to contain the same arrows in Sheet2, referencing the contents of Sheet1.
Or am I missing something?
The ArrowFormatter you found is useful if you accept to put your data inside a
Table visualization and host the visualization in a webpage, separate from your spreadsheet (I don't think there's a way to use formatters when embedding visualizations as gadgets inside a spreadsheet, unless you're willing to roll out some code of your own to do so).
-- R.