.. Opening the gate

You would need to find the key for the gate. I think you already did find a key somewhere. Do other stuff and look closely. Explore newly found areas.

Have you solved the tree maze?

You found the ramp between the trees, right?

Look closely while on the road after the ramp.

Look at the creepy sign. Interact with it to get the key from the sign.

Use the key you found from the creepy sign on the gate to open it.