Does not work for me. Neither if I’m using it as phx:gravatar and pasting the code or when I use it as gravatar-snippet. It produces no output.
For example I implemented it somewhere in a chunk of mine like this to test it:
As to tryout the phx:gravatar-snippet mode, I just used the code presented on the downloadpage in modx repository but neither happened a thing.

Perhaps someone can help me out? I am using modx 1.0.0 and jot 1.1.4.
Kind regards,
Alexander