Gekko can automatically run a strategy over the live markets and simulate in realtime what happen if you would have traded on its signals. Paper trading and backtesting are the two simulation modes that come with Gekko. It's a great way to experiment with strategies without putting your money on the line.
You can start a paper trader by going to live gekkos and clicking on "Start a new live Gekko".
Keep in mind that a paper trader is a simulation, and the accuracy depends on the market you decide to run it on (you'll get pretty accurate results on big markets like USD/BTC). You can read more about the details and limitations of the simulation on the backtesting page.