Gruber invented Markdown, full stop. If people want to come up with a 'better Markdown", fine but call it something else.
He may have selected the particular syntactic conventions it uses, but the idea of parsing ASCII markup and converting it to a formatter's input syntax is much older. I'm aware of prior art dating back to 1986 or so -- and wouldn't be surprised if that wasn't the first either.
Some people want to make a Markdown spin-off: clean, standardized, tested etc. Nothing ever stopped them, but they could really use the prestige given by the original name.
The question is, does John Gruber have the right to that name? I think he should give it away, but if he won't, we probably shouldn't force him. I'd change my mind if someone makes a compelling case against trademarks.