There are multiple areas in Pokemon Brilliant Diamond and Shining Pearl. One of those areas is the Lake Acuity and Snowpoint Gym. Below is a guide on how you can reach Lake Acuity and Snowpoint City.
Get the Strength Hidden Move first
Before going to the Snowpoint Gym, we need the Strength ability first. Go to the Solaceon Town and head to the Lost Tower. Climb up until you reach the top floor or 5th floor.
There are two NPCs here and you need to talk to the NPC on the left. He will give you the Strength Hidden Move.
How to Reach the Snowpoint City
Go to Eterna City first and keep going to the right until you reach Mount Coronet. Use Rock Smash on the rock and Strength on the second rock you encounter. Go up and move the boulders blocking your way using Strength.
Go down the staircase and use Defog in the next area. There is a Stardust here that you can get by breaking the boulder. Keep going north until you get outside the cave and reach Route 216.
Follow the route to the left until you reach Route 217, go north until you reach the Lakefront. Keep going right until you reach Snowpoint City.