I'm not so sure, the gantzers can see normal people. I think their must be a third "stealth" frequency. One where your not visible from any frequency. When Kei is rescuing Tae. or at least trying, he goes into this stealth frequency and neither Tae nor the rouge gantzers under izumi can see him.
I agree with that whole third frequency thing. I always wondered why nishi always wore his clothes over the suit though... maybe he was just worried that the other people would steal his clothes once the missions start but then again nishi would've probably loved to have reason to kill someone.