It's an error.
Here is the measure in question from the first edition of the Goldberg Variations.1 Note that it includes the A rather than G.
What establishes this as correct is the existence of the handexemplar — an annotated version of the first edition, in Bach's handwriting.2 Here is the same measure from that edition. Notice there are no annotations or corrections.
However, to reinforce things, Bach did make other annotations elsewhere in variation 25. For example, in the first measure, a tempo indication and several ornaments were added.
Here again is the first edition:
And the handexemplar:
Some additional corroboration is offered by the Henle Goldberg Variations, revised edition (1978), which was produced in light of the discovery of the handexemplar. The commentary on Variation 25 makes no mention of measure 9.3
1All images taken from ISMLP.
2The handexemplar surfaced in 1975 in a private collection and is currently held by the Bibliothèque Nationale de France. The annotations are discussed in detail in Christoph Wolff's 1976 article "Bach's 'Handexemplar' of the Goldberg Variations: A New Source", Journal of the American Musicological Society 29/2: 224–41.
3The Preface, which mentions the handexemplar, and the Commentary, can both be found on the Henle website.