gistfile1.scala

def compare = Action.async {
    implicit request =>
      play.Logger.debug("compare branches")

      val future: Future[(List[String], List[String])] = for {
        newRepo <- Gitweb.branches("reservit")
        oldRepo <- Gitweb.branches("components")
      } yield (Gitweb.extractBranches(newRepo.body), Gitweb.extractBranches(oldRepo.body))

      future.map { case (newRepo, oldRepo) =>
        val migrated: List[String] = newRepo.intersect(oldRepo)
        val notMigrated: List[String] = oldRepo.diff(newRepo)
        val onlyNew: List[String] = newRepo.diff(oldRepo)

        val futures: List[Future[Res]] = notMigrated.map { branch =>
          for {
            o <- Gerrit.open(branch)
            m <- Gerrit.merged(branch)
          } yield Res(branch, o.body, m.body)
        }

        Future.sequence(futures).map {
          results =>
            // I want to return this result but I don't find how :(
            play.Logger.debug(s"futRes $results")
            Ok(views.html.git.compare("Comparaison", newRepo, oldRepo, migrated, onlyNew, notMigrated, results))
        }
        // I wish I do not return this one
        Ok(views.html.git.compare("Comparaison", newRepo, oldRepo, migrated, onlyNew, notMigrated, List()))

      }.recover {
        case e: Exception =>
          play.Logger.error(e.getMessage, e)
          Ok(Json.obj("error" -> e.getMessage))
      }

  }

I think you have a Future[Future[Result]]. What you need is a Future[Result]
The first future.map should be a future.flatMap

You can have a look at https://github.com/scala/async
In my experience, it is easier for dev to use Future with this library.

I explicitely add types but I do not see a Future[Future[xx]] or maybe the Future.sequence in the future.map block is indeed a Future[Future[xx]]

The Future.sequence should return a Future[Result].
But it is itself inside another Future.map function (https://gist.github.com/gbougeard/86ae26acf7841fb161fb#file-gistfile1-scala-L10)

The first Future.map should be a flatMap

Yes first map should be flatMap...
Concerning Async, having studied it a lot, I can say it works but has limitations and I'm not a so big fan of this kind of approach... it depends on your way of developing actually ;)