Group.IssueIssue operates on a Group's issues. There is a separate User and Project issue module.
val issues : 
  token:Token.t ->
  group_id:int ->
  ?state:Gitlab_t.state ->
  ?assignee:string ->
  ?assignee_username:string ->
  ?milestone:string ->
  ?labels:string list ->
  ?author:string ->
  ?author_username:string ->
  ?scope:Gitlab_t.merge_request_scope ->
  unit ->
  Gitlab_t.issue Stream.tissues ~token ~group_id () returns a list of group issues.
See List group issues.