code: public List<Milestone> getGroupMilestones(Object groupIdOrPath, MilestoneState state) throws GitLabApiException {
          Form formData = new GitLabApiForm().withParam("state", state).withParam(PER_PAGE_PARAM, getDefaultPerPage());
          Response response = get(Response.Status.OK, formData.asMap(),
                  "groups", getGroupIdOrPath(groupIdOrPath), "milestones");
          return (response.readEntity(new GenericType<List<Milestone>>() {}));
      }
docs: Get a list of group milestones that have the specified state.
  
  @param groupIdOrPath the group in the form of an Integer(ID), String(path), or Group instance
  @param state the milestone state
  @return the milestones associated with the specified group and state
  @throws GitLabApiException if any exception occurs