Bret
This is two-pronged:
  1. Updating the "Avatar Letter" attributes seems to have no effect in Gist.
  2. Avatar Letters in Gist (for users without Gravatar profiles) display the first two letters of the user's first name. This is uncommon. The typical convention is one of the following:
A. First letter of first name.
B. First letter of first name + first letter of last name.
This is low priority, but I advocate changing to A or B, so it's more understandable to people providing support.