According to the article it was a contract. The University of Chicago entered a contract with the Pearson family to host an institute. That the university reported it as a gift in the press was perhaps unfortunate, but it is understandable as it has better optics.

If the idea for the Institute had been the universities, and they were soliciting donors to fund it, and Pearson was one of multiple donors, sure that would qualify as a gift, but as the article is written, this was a very different beast.