Modify gform_other_choice_value for specific form and specific field in Gravity Forms

The second parameter is the field which contains the form ID to which it belongs and its own ID so you can target a specific form/field like so:

add_filter( 'gform_other_choice_value', function( $placeholder, $field ) {
    // Update "123" to your form ID and "4" to your field ID.
    if ( $field->formId == 123 && $field->id == 4 ) {
        $placeholder="Your custom placeholder";
    }
    return $placeholder;
}, 10, 2 );