As a heavy gmail user (three accounts with different domain names for work, personal, and public), I've been late to the party learning the gmail keystroke shortcuts. For me, the hefty productivity benefits only derive from a small subset of the keystrokes, so it shouldn't be hard to learn the important ones.
But which ones are important to learn? I copied gmail's docs into a spreadsheet, and started marking the ones I thought most helpful for my particular work patterns. Then I focused on practicing just those few I marked.
This worked well for me, so I uploaded my gmail_shortcuts.xls spreadsheet as a convenience to anyone else interesting in learning using the same approach. The top row has dropdowns for filtering on whichever criteria interests you; I use the "status" column to design which keystrokes to ignore, and which to learn. Feel free to download and edit.