A 25% climb at 250 watts is going to feel the exact same as a 16% grade at 250 watts. The gear doesn't matter. The only variable will be your speed. You're still applying the same power to the pedals (250 watts) in both situations, you're just going slower on the steeper climb.

On a trainer, we don't have factors like the angle of the bike, available traction, or the difficulty of balancing at slower speeds. So the only difference is power vs speed. A 25% grade will only feel harder than a 16% grade because you either have to go slower (so it takes longer to cover the same distance) or you have to ride at a higher power output to maintain that same speed. If speed and cadence aren't a factor to you, then gradient is also irrelevant.

To get an idea of "how hard", you can calculate how much power you need to ride a 16% or 25% climb at a specific speed (Bike Calculator):

• A 160lb rider on a 20lb bike will need 250watts to ride a 16% climb at 4mph. A 5 mile climb at 16% using 250 watts of power output would take 75 minutes.
• That same rider on a 25% climb would either need to slow their speed down to 2.6mph at 250 watts, or increase their power to 385 watts to maintain 4mph. A 5 mile climb at 25% using 250 watts of power output would take 115 minutes, or it would take 75 minutes if you could maintain 385 watts.