How can I upload bulk images as separate nodes in drupal 8?
DropZoneJS can auto-create media entities for every uploaded image.

Remember that you need to set up an entity browser using the DropZoneJS-Widget for the bulk-upload feature.

Your image gallery is an entity reference field to media entities (unlimited cardinality).

If you are looking for a general tutorial on how to set up media entities, I found this tutorial quite helpful.