To use initials as the Gravatar default when the user does not have an account or e-mail configured, simply pass the d query string parameter containing the URL-encoded link to the image that corresponds to the user's initials:
https://s.gravatar.com/avatar/a5bfc9e07964f8dddeb95fc584cd965d?s=120&r=pg&d=https%3A%2F%2Fraw.githubusercontent.com%2Fdmhendricks%2Fgravatar-initials%2Fmain%2Fdist%2Fmulticolor%2Froboto-light%2Fgv.png
Images are original, created by me, released under MIT license. Use them for whatever you want. The examples above link directly to this repo, but you'll probably want to download the images and host them yourself.
Included characters:
- 0-9
- A-Z
- 00-99
- AA-ZZ
- A0-Z9
- 0A-9Z
This should account for any combination of single or double letters and numbers.