I have bit similar question to Optionally Using GPL Library via System API in Closed-Source Application but not 100% the same.
Let's say I have closed application A (i.e. frontend application). It is exposing API keeping rights to it, but grants open rights to execute it (code is closed, API is publicly available).
Then is app B (i.e. backend admin panel) which is GPL admin for GPL C (i.e. frontend application). If I fork B and add support for A API and publish back under GPL is it still ok?