Hello Daniel,
if you want to search for GIDs of entities within the api, you can use the GidAgent.findEntity method.
Based on your request, I have suggested that GidAgent.findIDProvider(<String of a GID>) (which is used to find IDProviders instead of entities) will also be included in the API.
It is important to know that these two methods are using separate "UID spaces". One method therefore does not find anything that the other method would find. Furthermore, it is not guaranteed that GIDs cannot be present in both "UID spaces" (even if this is extremely unlikely).
IMPORTANT: GidAgent.findIDProvider is not API. This method therefore may be subject to change! Please check the release notes of future FirstSpirit versions or submit a ticket to Customer Support to be informed when/if the ticket with the internal ID CORE-16394 is released.
Although you have mentioned other points, I assume that my answer is sufficient to fully resolve the issue raised. However, if there are any points still open, please let me know so that they are not overlooked.
Best regards
Holger