GLUCK: "IPHIGENIE EN TAURIDE." Diana Montague, John Aler, Thomas Allen, Rene Massis; Monteverdi Choir; Orchestra of the Lyon Opera, conducted by John Eliot Gardiner. Philips 416 148-2 (two compact discs). This eminently satisfying, first modern recording of Gluck's greatest Parisian triumph comes with a startling surprise. Instead of casting the conventional dramatic mezzo, Gardiner has used the lighter-voiced Montague, who may possess the basic equipment of a Zerlina, but wields also a sensitivity to the French declamatory style that lends her Iphigenia dramatic nobility without the taint of 19th-Century ponderousness. Montague's performance sets the pace here. Allen's noble Orestes, Aler's ardent Pylades and the trim, expressive choral work all help to propel this 1779 saga of reconciliation, revenge and godly deliverance to its radiant conclusion. Gardiner's stylish, balanced, alert contribution with the medium-sized Lyon orchestra is the recording's most felicitous asset.