I'm also having problems with an invisible Gerevick.
Replacing the appearance.2da file with the original TSLRCM did nothing, and neither did removing it (I got a backup, don't worry).
The only other mods I've got installed are hotor 1.6 and Realistic Visual Effects, both installed using that TSL patcher.
Weirdest thing is that he appeared normally in Khoonda before going into the enclave.
My graphics card is the ATI Radeon HD4870 1GB version on WinXP SP3. (Although I doubt that has any influence.)
The HK-50 series is also partially transparant (at least on Dantooine), quaint, but it doesn't bother me.
And the game crashes after killing the last laigrek in the enclave mid-section (straight ahead from the entrance).