How did Glenn Gould know, in 1955, the proper ornamentation for Bach's "Goldberg Variations"?

Question

_{This question has its origin in Why does the Open Goldberg score have a G rather than A in bar 9 of variation 25?. See that Q&A for background information.}

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At what time, and on what basis, was ornamentation corrected in the Goldberg Variations, Variation 25?

In the first edition of the Goldberg Variations, the first measure does not contain an ornamental D5 preceding the Bb5.

However, in Glenn Gould's 1955 recording, he includes the ornament.

The ornament does appear in the handexemplar, but that was not discovered until 1974.

How did Gould, in 1955, know about this ornament?

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According to the editions available on IMSLP, the ornament was included as early as Czerny's ca. 1850 edition. But at what point was it first introduced, and on what basis?

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Wanda Landowska, in 1933, made the first Goldberg recording. She does not include the ornament.

Answer 1

Apart from being in other readings as you noted, this is a typically baroque portamento ornamentation that lyrical solos (like those executed by singers) would quite often be expected to include by default. With an ornament immediately before and a long target note after a jump upwards, this is quite the location for this ornament.

Here the lead voice is clearly in the lyrical ballpark, so giving it this ornament is sort of a natural execution choice in baroque practice. Whether Gould made this choice independently of other readings or in acknowledgment of them is likely impossible to discern at this point of time.