CLASSICAL Gershwin, George: ''Songs.'' Barbara Hendricks, soprano. Katia and Marielle Labeque, duo-pianists. (Philips 9500 987) - Barbara Hendricks, one of America's finest young sopranos, tackles this classic American literature with grace, insight, sumptuous sounds, and a fine sense of words and mood. The Labeque sisters are French, but they love Gershwin, and their arrangements in the accompaniments are always interesting, if a bit on the hybrid side. The majestic sobriety of classical and the rhythmic freedom of jazz do not always make an ideal partnership. Therefore, those who feel that Gershwin songs belong strictly to jazz singers may not care to sample this album. The songs included here are ''The Man I Love,'' ''They Can't Take That Away from Me,'' ''I Got Rhythm,'' ''Summertime,'' ''Has Anyone Seen Joe?'' ''I Loves You, Porgy'' and a medley of ''Love Is Here to Stay,'' ''But Not for Me,'' ''Embraceable You,'' and ''Nice Work if You Can Get It.'' In all, this is an enchanting disc, handsomely recorded in the analog (nondigital) mode.