Gemini Error lets you know details to share with support@bigbusiness.com on errors. 28 is a timeout error, the request took too long, but sent in relayed cryptic error codes.
Pictured is a connection timed out, which is not uncommon given Gemini's popularity. Making the request again worked fine, the problem being a standard request takes time.
Of course the error message does not say that in so many words and is instead relaying where the error was encountered (curl), an error number (28), and module (gem_post).