This was made possible by a combination of a highly detailed and explicit form of musical understanding, astonishingly intense and sustained powers of concentration, and a superb technique.
This goes beyond, is at a higher level than, the question of musical 'interpretation' (with such concerns as tempo, phrasing and fidelity to the score or composer's intentions) - it is simply the primary realisation of music.
Gould was not the greatest pianist - rather he was, in his fashion, the most complete performing musician.