Correct, GELF is a specific format put into JSON.
Do I sniff and rewrite in a pipeline preprocessor maybe?
Nah, pipelines etc come after receiving the message on an input. So if the GELF input rejects the message, it won’t go into any pipelines or extractors either.
I do believe you can fire up a generic HTTP receiver and tack a JSON extractor onto that. That’d do the job.