Converting calorie count on standard gym stationary bike to power data
Calculating calorie burn from power output is pretty straightforward, but the reverse simply cannot be done. The same number of calories can be burned by doing an hour of 30 second efforts and 2 minute recoveries compared to doing a one hour steady endurance effort. Beyond that,  you also have the inaccuracy of the gym bike measurement which will be the biggest factor to consider. Any bike which does not display power in watts to you also doesn't measure it in a way that is accurate enough to be useful in the slightest when comparing to data from any other unit. 

In short, you can use the calorie number to compare between workouts on the same gym bike to know that you're improving if you burn more calories in the same time at the same level of exertion, but nothing more than that.