'504 - Gateway Timeout' when Indexing the items

When Indexing the items, it fails sometimes and it gives,

The remote server returned an error: (504) Gateway Timeout. [The remote server returned an error: (504) Gateway Timeout.]

The Indexing logic is here as below, 

    var client = EPiServer.Find.Framework.SearchClient.Instance;
    List<ItemModel> items = getItems(); // Get more than 1000 items
    List<ItemModel> tempItems = new List<ItemModel>();
    //Index 50 items at a time
    foreach(var item in items)
    {
        tempItems.Add(item);
        if (tempItems.Count == 50)
        {
            client.Index(tempItems);
            tempItems.Clear();
        }
    }

What causes this to happen ?

Additional info:
EPiServer.Find.Framework version 13.0.1
EPiServer.CMS.Core version 11.9.2