He is the inventor and thought leader behind an agile-like methodology named EVO (for evolutionary).
In my own book, "Project Management the Agile Way: making it work in the enterprise", I include a lot of material taken from Gilb's work. I like what he says, though frankly a better spokesman for what Gilb advocates is Niels Malotaux: he writes with clarity and simplicity, something that eludes Gilb himself.
Recently, I fell onto a string of Gilb'isms, starting with a 2004 keynote address to an agile conference entitled "What is missing from the conventional agile and eXtreme methods?" His answer, in one word: quantification. And by this he really means quantification of business results, outcomes generated by project outputs, in business terms. He more or less sums up the idea with the words value and quality.
He really takes agilists to task for promoting the idea of a single customer/user as the arbiter of product requirements; instead, agile teams should be consulting--weekly--with the business stakeholders, a diverse group Gilb says can be more than a dozen in any real enterprise.
And, he has heartburn with user stories and other low level practices, like TDD, that he says unnecessarily constrain the inventiveness of developers and leads to "amateur" engineering. Gilb's formula: write down the business quality values on not more than one sheet of paper, and let the developers go from there.
In 2010, he wrote a couple of magazine articles in "Agile Record". The first part, entitled "Value driven principles and values" is a better explanation of what was in the 2004 keynote. The second part, entitled "Values for Values" builds out his ten principles--first presented in the keynote address--and given also in the first part article.
Gilb's principles are ones we can live by. Here's a reproduction of one version:
As Gilb says of the Agile Manifesto: "it's heart is the right place". Even if you don't buy all that brother Tom says, his heart is in the right place: better quality and value for stakeholders through projects and software.