I love all five of those quasi-ad lib movies (for lack of a better phrase) he wrote and acted in (I think directed as well). Maybe there are more then five but that is all I have seen. I would rank them this way...

1. Best in Show 9.0
2. Spinal Tap 8.5
3. A Mighty Wind 8.0
4. For Your Consideration 7.5
5. Waiting on Guffman 7.5

Do you know if Jane Lynch had improvised Witches in Nature's Colors in A Mighty Wind? That was hilarious. Just.... winc.

"Mockumentary" is the word that is generally used to describe them. Spinal Tap started them off which he costarred in and cowrote. Rob Reiner directed and had the idea for it. CG also did one for Netflix's called Mascots but that is all. I don't think he can get the financing for another in the style.

His first job directing a film is a good one too. It's called The Big Picture from 1989 and stars Kevin Bacon as a film student trying to make in in Hollywood. CG cowrote it and has a small role.

I know rock fans love Spinal Tap but wonder how say how real folk music fans or dog owners take to AMW or BiS. If you are too close to the subject he is mocking you might not find it so funny. Anyways thanks for bringing this up.