Sushi Izakaya Gaku   In-the-know izakaya addicts already flock to Sushi Izakaya Gaku, opened last year by chef-owner Manabu Kikuchi, a 10-year veteran of Imanas Tei, but we just discovered it, and we're hooked. To say that eating at Gaku is like being a kid in a candy shop may be a cliche, but that really is the feeling that overcomes diners. Do we really have to choose between spicy negihama tartare and the luxuriously textured natto, 'ahi, uni and yamaimo with two kinds of egg (ikura and quail)? The negihama tartare (a mixture of finely chopped fish, mayonnaise, sambal, sesame oil and a "secret ingredient," according to the sushi chef, who looks strikingly like Speed Racer) is topped with tobiko and a raw quail egg; as you bite through the nori crunch, you experience bursts of roe as your tongue plies through the creaminess. With Charles Lloyd, John Coltrane and other jazz masters on the speakers in the woody interior of the small room, you feel like you're a member of a low-key private club — and boy does it feel good.  1329 S. King St. 589-1329. 5-11 p.m. Mondays-Saturdays. JCS, MC, VS. No checks. $-$$$.