Michael Gielen is one of the more interesting conductors of his generation, though not one of the best-known. Like Pierre Boulez, he's an avant-garde composer who also conducts, but unlike Boulez, a superstar as composer and conductor, Gielen was more in the mold of the Kappellmeister who produces respected compositions (I've heard one of them; it was OK, similar to Boulez but without the orchestral appeal that make Boulez's serialisms tolerable to people who, like me, don't "get" serialism) and, as a performer, starts in the regional opera houses and works his way up. Gielen's big break came in the late '70s when he became music director of the Cincinnati Symphony Orchestra, but his repertoire choices and conducting style weren't to the taste of the orchestra or the audience; his tenure there was short, and he never had another post at a big-city orchestra. But starting in 1986, he took over the South-West German Radio Symphony Orchestra of Baden-Baden, a very good "provincial" orchestra that specializes in astringent modernistic music of the kind Gielen likes. And because the orchestra has its own recording studio and a deal with state-subsidized radio, Gielen has wound up making more recordings of standard repertoire music than many better-known, better-publicized conductors.
Many of these recordings are available on the Haenssler label, including recordings of The Complete Symphonies of Mahler. This might, overall, be the best Mahler cycle ever recorded (which is not to disparage individual favorite recordings by the likes of Bernstein, Kubelik, Walter, Levine, Chailly, etc). The sound is very good throughout -- all but one of them are studio recordings -- and Gielen is sort of like Pierre Boulez with heart: a lot of attention to detail, an emphasis on the "modernist" elements in Mahler, but no holding back or embarrassment about Mahler's schmaltzy moments, like the Klezmer music in the third movement of the first symphony, or the fifth symphony's adagietto (which Gielen takes very fast -- under nine minutes -- in keeping with the kind of tempo Mahler reportedly preferred for this movement). The set is only available from Germany at the moment, though some of the individual releases came out in the U.S. and Canada; the recording of symphony no. 7, on one disc, is probably the best version of that symphony on any label.
Other Gielen recordings I've heard and liked are his recording of Tchaikovsky's Sixth Symphony (coupled with two other really depressing pieces: Berg's Three Pieces For Orchestra and Ravel's "La Valse"), a sort of Mahlerian approach to a piece that probably influenced Mahler's later work; his fast and furious version of Janacek's Glagolitic Mass, and some of his Beethoven; his recording of the "Pastoral" symphony has the fastest first movement I've ever heard in this piece. Very interesting conductor, with a very interesting discography; David Hurwitz has a review of the Mahler box at Classics Today, which is even more enthusiastic than mine.