Based on my reading of the gmp-bugs mailing list over the past several years as well as the source code to various GMP releases, it is abundantly clear to me that Torbjörn is a highly talented individual and that the the GMP library would not be what it is today without him.
However, I must concur with Ludovic in this matter. I have seen many examples of this type of conduct from him, both towards myself as well as others. I am a software engineer who works in Silicon Valley, and I cannot imagine working with someone who is frequently abrasive and disparaging. Such an individual would not last very long on my team. I am well aware of the stress involved with writing and maintaining a complex piece of software, as well as working with people who are less technically competent than I am, but that doesn't give me carte blanche to be hostile towards them.
The tone of this community is determined in large part by its core developers. Niels Möller, for example, is consistently a great example of a positive, constructive team member.