Just to clarify this statement to avoid any confusion:
I've been asked via email to add support for GD3 to Mp3tag. I'm personally not interested in developing this feature and want to focus my spare hours on the core development of Mp3tag. So I've asked the OP to ask on the forums whether someone is interested in developing a tag source for this data source.
Please avoid any meta-discussion whether supporting this data source is reasonable or not -- simply jump on the train in case you like to support it and have the skills of adding support for it to Mp3tag.
Kind regards
Florian