code: public void deleteGroupMember(Integer groupId, Integer userId) throws IOException {
          String tailUrl = GitlabGroup.URL + "/" + groupId + "/" + GitlabGroupMember.URL + "/" + userId;
          retrieve().method(DELETE).to(tailUrl, Void.class);
      }
docs: Delete a group member.
  
  @param groupId the group id
  @param userId  the user id
  @throws IOException on gitlab api call error