I'm using a classic trainer with the old Garmin gsc10 speed cadence censor.

I don't know if it is the trainer, but I am pedaling harder in easier gears to maintain a slower mph than what I do on the road.

Sat, I rode 2 hours for an average of 18 mph and it is was a pretty easy pace.

On the trainer, my gears are easier and the speed censor is showing 14.5 mph and my legs are trashed.

Is it normal or is the calibration off? I entered my wheel size in the data.

The GSC-10 is very particular about the placement of the magnet. If that magnet has moved at all, everything goes bonkers. I also had issues when if the NDS crank arm magnet passed the GSC at the same moment the spoke magnet did, the GSC would get confused, and not know to log it as speed or cadence. This is the reason I abandoned my GSC-10 years ago in favor of magnetless.

The simple, and probably correct, explanation is that the resistance on your trainer is higher than what you see on the road. It might be something wrong with the trainer or it might be how much you've cranked on the roller pressure - hard to say.

If you really want to test to see if the sensor is reading differently, pick a gear combo and a cadence and try it both inside and out - my bet is that the speed reading will be the same but the trainer will feel more difficult.

It is the trainer as mentioned above. If you are pedaling at the same cadence in an easier gear the speed has to be lower even if it feels harder. With few exceptions, trainer speed has no correlation with outside speed