With Gilbert, from the start of the "Palomar" stories onward is a pretty safe bet. There's plenty of more surrealist things like "Errata Stigmata," but those are one-offs(Errata always seemed to me a friendlier ERASERHEAD). It's the Palomar stories that are the real meat of his work, especially his masterpiece "Human Diastrophism," and there's a big collection of 'em available that must be in softcover or used by now.
Now, Jaime, that's slightly more difficult, for me especially because I wasn't as much a fan of his stuff, pretty as it is. But I'd say that you want to start somewhere a little before "Death of Speedy." Basically, anything after Maggie is no longer in a fannish parody of 50s sci-fi with dinosaurs(that may sound like fun, but really, they're kind of clumsy and tedious stories compared to his later work); when she and Hopey are in the barrio, on the street, in the normal world, it's from there onward you want to go.