Legends may be shattered tonight when a one-hour television special, "Glenn Gould Plays Beethoven," airs on PBS (Channel 26, approximately 11).

The great pianist, who died of a stroke shortly after his 50th birthday last year, is enshrined in the memories of thousands of concert-goers primarily for his eccentricities, musical and personal: the low piano stool, the humming at the keyboard, the wearing of overcoat and muffler while performing on hot summer days. This program focuses on his musicianship, which is equally spectacular but not as colorful.

The young man who appears on this tape (made by the Canadian Broadcasting Corp. in 1970) is certainly eccentric, but only occasionally does he live up to the legendary stereotypes. Perhaps he mellowed in the years after his retirement from concerts in 1964, or perhaps he was less eccentric without a live audience.

His gestures, often in a real-time slow motion, are not those of any other pianist. Sometimes he moves his arms and shoulders with the curiously graceful adagio effect of someone walking underwater. His facial expressions may display agony or ecstasy; all that can be said for sure is that they are intense. His left hand sometimes seems to be conducting his right when it has no notes to play for a few bars. But he is dressed in a normal business suit with no cold-weather extras; he does not slouch or slump more than other pianists.

He can be seen humming along with the music (or perhaps singing or counting), but he cannot be heard--at least on a television set played at moderate volume--the way he can on many recordings. From this viewpoint, low-fidelity sound may be an advantage, though it is not when an orchestra is brought in (the Toronto Symphony, conducted by Karel Ancerl) for Beethoven's "Emperor" Concerto.

Musically, this "Emperor" is remarkably straightforward for a Gould interpretation. There are some unusual accents and some moderate fiddling with tempos, but they are hardly as noteworthy as his superb technique. In two brief solo works, Gould is more his individualistic self--particularly in a Bagatelle from Opus 126 taken at a snail's pace. But this and the Variations in F, Op. 34, are notable chiefly as examples of the pianist venturing outside of the repertoire (Bach and 20th-century music) that was most naturally congenial to him.