From opera stage and keyboard; Showcase of voices - and a Horowitz recital

The ''Gala Rossini Festival'' that Carnegie Hall put on as a showcase for the formidable talents of Marilyn Horne has come to a glorious end with her singing the title role in ''Tancredi.''

The trio of operas in the series have received performances that any major opera house would be proud to accept as their regular standard, even though each performance was marked by a major cancellation.

Miss Horne was in fine form, growing in strength right through to the protracted death scene, in which she held her audience in a spellbound hush. The combination of distinctive timbre, remarkable voice, and extraordinary artistry fuses to make Miss Horne one of the legends of this or any age.

These three evenings have offered the thrill of hearing a great voice used to its fullest potential. Her pyrotechnics are of the sort to make one gasp, but in the introspective moments, Miss Horne becomes even more exceptional. Not only does she spin out seemingly endless lines, she suffuses them with an adept balance between beauty of tone and capturing the right emotion through shading and color.

''Tancredi'' is not the strongest of the three operas offered. The plot is rather silly, involving Tancredi's stubborn refusal to admit that his love, Amenaide, is true to him. That stubbornness leads to his demise, in true operatic fashion. Plot triteness notwithstanding, ''Tancredi'' received by far the most even performance of the ''cycle,'' solidly cast from top to bottom. Ralf Weikert conducted the excellent orchestra. He understands Rossini, and he understands singers, which makes him a rare combination. Although solicitous of his singer, he at no time sapped the dramatic content of the score.

Bass Justino Diaz and mezzo Rose Taylor sounded ill at ease in their assign

ments, but otherwise standards were very high. In the lesser role of Roggiero , Patricia Schuman showed great promise. Lella Cuberli, an American soprano who has spent most of her time in Europe, sang Amenaide. This summer she repeats her role in Venice and for CBS/Fonit-Centra records (with both Miss Horne and Mr. Weikert).

It is not that Miss Cuberli has the most beautiful of voices - in fact it tends to hardness at full volume in the upper extremities - but that she uses the voice in stylistically authentic fashion. She knows how to sculpture phrases , how to use shading and dynamics to enhance the emotional content of the music. In the quiet singing there are some ravishing timbres to be heard. A certain archness mars her stage presence, and clearly the voice is not meant for heavy roles (even here she sounded somewhat ''overparted''), but she is a real discovery.

Chris Merritt is vastly improved from the days of his promising debut at the New York City Opera. The voice has taken on a solidity in the lower reaches without marring the flexibility. His stratospheric high notes have real quality and ring. He found in the role of Argirio plenty of opportunities to show off his voice as well as his considerable artistry.

Next season the series turns to ''French Opera Comique'' as a showcase for Frederica von Stade. All three operas are rarely heard. One, Massenet's ''Cherubin,'' is virtually unknown. The two others, Thomas's ''Mignon'' and Offenbach's ''La Pericole,'' have fallen out of favor. Joining Miss von Stade for these evenings will be such singers as Nicolai Gedda, Donald Gramm, Neil Rosenshein, and Samuel Ramey. Weak recital by Horowitz

Vladimir Horowitz's Metropolitan Opera House recital two weeks ago proved profoundly disturbing.

No matter what one thought of the virtuoso as a musician, his prodigious mastery of piano playing for years verged on sorcery. Though recent encounters had shown signs of some reduction in consistency and accuracy, the playing, for the most part, remained the stuff of legends.

Last year he performed in London for the first time in 31 years, and, judging by the live telecast, much of the magic was still there. Two weeks ago at the Met, there was unhappily little to recommend itself. A capacity crowd had spent up to $100 a ticket at benefit prices to hear this pianistic superstar. Who would have suspected that a player of such high standards would have allowed himself to appear in dramatically reduced form?

The program was a demanding one - Beethoven's Sonata No. 28, Op. 101, Schumann's ''Carnaval,'' and an all-Chopin second half. Whereas occasional wrong notes of themselves should be no important distraction, the Beethoven and Schumann were so error-ridden as to be at times unrecognizable. Musically, there was no thread or mystery or even warmth in either piece.

The Chopin found Horowitz fighting his own past triumphs: What one was left with was playing that did not accomplish those very things upon which his career triumphed so electrifyingly for so long. And for the first time since I can remember, he played no encores - it was as if he knew the evening had not gone well.

Happily, we have the records - including many live RCA discs from recent years - to keep the image of Horowitz, tonal wizard, forever in the present. Those records have ensured his place in the pantheon of piano immortals. Giulini TV concerts

Two more Giulini concerts will be aired tonight and next Wednesday (The Giulini Concerts II, PBS, check local listings). These performances were taped at the Dorothy Chandler Pavilion, home of the maestro's Los Angeles Philharmonic.

They continue the high standards set in the two programs taped in Japan that aired earlier this season. Tonight's Brahms First is a deeply felt, substantial account of that work. It captures the brooding mood impressively. Any chance to watch Giulini's face close up is a particular pleasure, so completely does it reflect the mood of the moment.

The exceptional program, however, airs next week, when Murray Perahia and maestro Giulini explore Beethoven's Second Piano Concerto. Then the orchestra plays Hindemith's ''Concert Music for Brass and Strings.'' The former is a marvelous meeting of youth and experience. Mr. Perahia has always been one of the younger generation's most thoughtful and provocative pianists, and Giulini brings out even more subtlety and plangency than usual.

Hindemith's ''Concert Music'' receives an impassioned, blazing performance without the harsh stridency most conductors find imperative in this music. Giulini gets the L.A. brass to play with liquid tone and mellow, burnished hues - some of the most consistently beautiful brass playing I've heard on records or in the concert hall.