Looks pretty efficient to me -- a bunch of people decided they wanted some and the supply is limited, so prices rose?
That's not really how it works. The efficient-market hypothesis postulates that a stock's price reflects all of the relevant information that could affect its value. You might expect a stock's market capitalization - that is, the cumulative value of all of its shares - to be the same as the value of its net assets. If that were true, then that'd mean that you could theoretically purchase a company for the same price at which you could sell all of the company's tangible assets (i.e., physical & intellectual property).
However, almost no company's market cap is its net-asset value because there are additional factors to take into consideration, such as the company's potential for growth, the leadership of the company, R&D spending, etc. The efficient-market hypothesis holds that a stock's value is actually a reflection of all of these harder-to-measure factors, thereby determining a company's value. What GameStop shows is that a bunch of people can just dramatically inflate the price of a stock by buying a bunch of it on a whim as a meme, thereby demonstrating that stock price isn't necessarily grounded in reality.
