Fires on the Form Editor page after a field’s settings panel has been opened and settings have been displayed.
The gform_field_standard_settings and gform_field_advanced_settings hooks are responsible for displaying custom field setting UI. These hooks are typically used in conjunction with the gform_editor_js hook to output any JS required by the custom settings UI. As part of this process, the gform_load_field_settings JS hook can be used in the gform_editor_js PHP hook, to only perform certain actions once the field settings have loaded.
This filter is located in gravityforms.js