I'm not fully clear either, the story on his site is a bit lacking in specifics.
The claims made are:
- People are forking his code. This is legal by the GPL and he says explicitly he's fine with it.
- People are releasing this forked code commercially. This is also legal by the GPL and again he explicitly says he's fine with it, in facts encourages it.
- (What isn't clear is whether these people are releasing their forked source. If not, that's a GPL violation.)
- People are renaming the software in their forks. This is legal by the GPL, IF you clearly indicate your version is a fork of Mayan. The author states he is unhappy with this, which is quite understandable but not grounds for legal action. From his phrasing I conclude that he also believes it is not a complaint with legal implications.
- (It isn't clear is whether these people are actually clearly marking their fork as a derivative.)
- Apparently at least some of the people concerned are suggesting that the original software they forked from (i.e. Mayan) is abandoned. Such a suggestion, I assume, is made to encourage adoption of the fork. As far as I can tell, that is legal by the GPL, but may well be illegal as misrepresentation in sales. Again, and quite understandably, the author states he doesn't like this practice.
So there appear to be two possible GPL violations here: failing to deliver the source of a fork and/or failing to clearly mark a fork as a derivative work. Additionally there might be unlawful misrepresentation.
That's what might possibly be the case. What is actually the case is impossible to tell without additional information.