We're actively working on increased documentation for Gen, but for now the best thing is to look at the examples in examples/gen and examples/jitter-examples/gen, and the Gen Overview vignette in the reference (Help menu -> Max Help, click on 'Vignettes' in the side bar, and then on 'Gen'; or simply open the gen~ or jit.gen help patcher, click on the '?' tab, and select "Gen Overview").
It's almost identical to the 'Gen Overview' that Graham Wakefield mentioned, but when i look at the 'Gen Overview' doc/vignette, i notice it's missing the helpful codebox/'GenExpr' section that the .pdf has. Is this section found elsewhere in the included help these days? (or was it cut out entirely? if so, it really helped me out and thought i'd mention i feel it should be included somewhere. there are some extra sections i couldn't find anywhere like 'background', 'language basics', 'multiple return values', 'defining genexpr functions', etc...)
What you'll find there is that material, relocated. And Wesley's probably too humble to mention that nearly everything about the current Gen documentation has improved, and is improving (I am particularly enjoying the ability to option-click on a Gen operator to receive a compact and succinct description of the operator's function. Bravo!). As for further tutorial materials, please stay tuned....
What I'm wondering is if the GenExpr language will ever have a complete manual so that someone with no experience with text-based coding could start from scratch with it, or if GenExpr's similarities to C and Java make it so learning the fundamentals of those languages is the place to start.