I just saw this 1968 documentary on Turner Classic Movies. While I learned some interesting things about this director as well as the way he made films, I was surprised how poor the overall production was. On the plus side, showing film of Gance and his cameramen shooting some actual scenes was amazing--particularly when they discussed his amazing innovations. However, on the negative side, there was the very dull narration (both the voice and script), the film seemed to lack depth and told us practically nothing about Gance himself, and finally because it was so incomplete a record of Gance's films. It was all just so superficial and dull and inadequate. Some of the incompleteness is not the fault of the documentarians, but they mention several "lost films" of Gance's that were shown immediately after the documentary on TCM--and fully restored. This recent restoration begs the question "why doesn't TCM now make a documentary given that they now have better and more complete film stock?".
My advice--you'd do better to do an internet search on Gance or find a book about him. This film just doesn't do much to inspire, enlighten or entertain.