I won financially. Encyclopedia sets were expensive.

Your savings are real, but they don't show up in GDP or a profit-and-loss statement of any company.

The cost of buying the encyclopedia becomes disposable income to be used on other consumer goods. In that sense in shows up in lots of other companies profit-and-loss statements.

Maybe, but that's very diffuse and hard to attribute to Wikipedia.

And it would show up in real GDP, not necessarily in nominal GDP.