You can either use the actual and scheduled start and finish dates/times to do this. These columns are automatically visible in gantterAi when you create projects. 

Or you can create a baseline. Go to baselines on the top tool bar then click "manage baselines". Go ahead and add a baseline. I recommend setting a color for the baseline to better view the column. Then hit SAVE and go back to Baselines in your top toolbar. 

A Baseline is a snapshot in time for the time you add the baseline into your project. You can have multiple baselines to track the differences between your actuals vs your baselines as your project progresses.