The  wpcrm_system_gdpr_contact_fields filter is passed an array of default contact fields:

$data = array(
     'prefix'         => get_post_meta( $id, '_wpcrm_contact-name-prefix', true ), 
    'first_name'        => get_post_meta( $id, '_wpcrm_contact-first-name', true ),     
    'last_name'        => get_post_meta( $id, '_wpcrm_contact-last-name', true ),
     'organization'        => $organization,
     'role'            => get_post_meta( $id, '_wpcrm_contact-role', true ),
     'street_1'        => get_post_meta( $id, '_wpcrm_contact-address1', true ),
     'street_2'        => get_post_meta( $id, '_wpcrm_contact-address2', true ),
     'city'            => get_post_meta( $id, '_wpcrm_contact-city', true ), 
    'state'            => get_post_meta( $id, '_wpcrm_contact-state', true ),
     'postal_code'        => get_post_meta( $id, '_wpcrm_contact-postal', true ),
     'country'        => get_post_meta( $id, '_wpcrm_contact-country', true ),
     'phone'            => get_post_meta( $id, '_wpcrm_contact-phone', true ),
     'fax'            => get_post_meta( $id, '_wpcrm_contact-fax', true ),
     'mobile'        => get_post_meta( $id, '_wpcrm_contact-mobile-phone', true ),
     'email'            => get_post_meta( $id, '_wpcrm_contact-email', true ),
     'url'            => get_post_meta( $id, '_wpcrm_contact-website', true ),
     'information'        => get_post_meta( $id, '_wpcrm_contact-additional', true ),
     'categories'        => get_the_terms( $id, 'contact-type' ),
     'comments'        => get_comments( array( 'post_id' => $id ) )
 );

You can modify this array if there is certain information that you want to have added or removed from the array.