I understand I need to pick up a weapon with the attachment to unlock it, but where the hell do I find one? I've played through the game twice and haven't found it.

I know I got it right before your first encounter with the Ceph Tri-Pod mech. Make sure to look around and use your binos!

Thanks. I'll really scour the area next time. Which chapter is the first time you fight one of those? Is it the train station?

I never picked up a weapon with the Gauss attachment. I did, however, pick up a Gauss Rifle. After that, I noticed that I had the attachment unlocked.

I never picked up a weapon with the Gauss attachment. I did, however, pick up a Gauss Rifle. After that, I noticed that I had the attachment unlocked.

I've gotten the gauss rifle multiple times but the attachment remains locked.