It isn't possible to use Gravatar && filter the "avatar default"

[henry.wright]

If Gravatar is disabled using the bp_core_fetch_avatar_no_grav hook, the bp_core_default_avatar_{object} hook is executed. Therefore, the latter hook allows us to filter the "avatar default" when Gravatar isn't used.

It isn't currently possible to use Gravatar and filter the "avatar default".

Ref: ​https://github.com/buddypress/BuddyPress/blob/master/src/bp-core/bp-core-avatars.php#L572

[henry.wright]

Attached is one possible solution. It allows the default Gravatar src value to be filtered (the d parameter).

[henry.wright]

Think I'll close this request because an extremely small number of people will need such a filter. Please feel free to reopen if needed.