The best way to simulate lower gearing on your trainer or smart bike is to adjust the Slope Scaling settings in FulGaz.
This allows you to adjust the trainer "feel" for both uphill and downhill. Lowering your Slope Scaling Uphill is like replacing your 25T cassette with a larger cog. It allows you to spin up the hills rather than grinding at a low cadence. It does not change your speed in relation to your power.