Which metadata standard introduced the unofficial 'XMP Data' GIF application extension?

The metadata standard associated with the unofficial “XMP Data” GIF application extension is the Extensible Metadata Platform (XMP).

XMP was created by Adobe as a framework for embedding standardized and custom metadata in digital documents and media. It defines a metadata model, serialization format, and core properties, allowing information such as authorship, editing history, and descriptions to travel with a file.

GIF application extensions are blocks that let software add application-specific data beyond the core GIF image and animation structures. The “XMP Data” identifier and “XMP” authentication code provide a way to place XMP packets inside a GIF file, although this is not part of the original core GIF89a specification.

XMP is often confused with IPTC or Exif. IPTC supplies widely used news and photo metadata schemas, while Exif records camera and image-capture information. XMP can carry or reconcile metadata from those systems and can also be embedded in formats such as JPEG, PDF, PNG, and GIF.