See Houssaye, p. 205, for the sequence of these events.
Ollech, pp. 167-171. Colonel Basil Jackson, in his "Waterloo and St. Helena" (printed for private circulation), p. 64, states that he had been employed in examining and reporting on the Belgian roads, and did so on the road leading south from Wavre. This report had been sent to Gneisenau, and must have given him greater confidence on the night of the 16th.
O'Connor Morris, p. 176, approves Napoleon's criticism, and censures Gneisenau's move on Wavre: but surely Wavre combined more advantages than any other position. It was accessible for the whole Prussian army (including Bülow); it was easily defensible (as the event proved); and it promised a reunion with Wellington for the defence of Brussels. Houssaye says (p. 233) that Gneisenau did not at once foresee the immense consequences of his action. Of course he did not, because he was not sure of Wellington; but he took all the steps that might lead to immense consequences, if all went well.
Müffling, "Passages," p. 238: Charras, vol. i., p. 226, discredits it.