How can GameStop be short 140% of float?

TomTom is right, for the most part, that the float does not include everything outstanding.

However, in the case of GameStop, that doesn't explain all of it. As you can see from this Morningstar article, on 12/31/2020, Gamestop's total Shares Outstanding (not the float) was 69.75m shares, but there were 71.2m shares sold short at that point (the Float at that point was much lower, 27.3m).

As this Motley Fool article explains, the same share can be sold more than once, if the "buyer" of the short-sold share then lends that share out for shorting:

As an example, take a situation involving four investors. Annie owns shares of GameStop, and Annie and her broker have an agreement that allows the broker to lend Annie's shares to short-sellers. It lends them to Bob, who subsequently sells those borrowed shares short in hopes that GameStop's share price will fall.

An investor named Chris ends up buying those borrowed shares from Bob. However, Chris has no way of knowing that those shares have been borrowed from Annie. To Chris, they're just like any other shares.

More importantly, if Chris has the same kind of agreement, then Chris's broker can lend out those shares to yet another investor. Diane, another GameStop bear, can borrow those shares and sell them short.

In this example, the same shares end up getting borrowed and sold twice. The short interest volume these transactions add to the total is twice the number of shares actually involved. You can therefore see that if this happened throughout the market, total short interest would eventually exceed the number of shares outstanding and approach 200%.