Sorry for the delay in responding - When you execute a LineTrace against the scene, you can choose to trace against the simple collision bodies or the complex collision bodies.
A static mesh supports configuring both complex and simple bodies.
Simple exists because it is faster to query against than complex.
Complex exists because it is more accurate than simple. (complex == use actual trimesh)
To be clear - Physx injects all the collision bodies into a hierarchical spatial acceleration structure (e.g. octree).
This structure is designed to rapidly reject or cull non-relevant bodies.
Due to this , complex collision is still fast - just not as fast as simple collision.
For your case, you can try setting complex collision on the static mesh of the gate and then use LineTraces with complex collision for the bullets.
Profile and see if it is fast enough for you game.