There is no property for hiding the RadImageGallery ThumbnailsArea so setting Height and Width to zero should be enough. Additionally, the loading animation represents a RadAjaxLoadingPanel which indicates the next image loading. You could optionally hide it by setting the RadImageGallery.LoadingPanel.Visible property to false.
In order to bind the RadImageGallery from a PageMethod you could retrieve the data loop through the returned items and manually create each of the RadImageGallery items as shown in the client-side binding demo.
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