A trick I’ve tried when posting for clients and dealing with washed out H.264 exports:
After you’ve exported the Quicktime, open in Quicktime player. Go to Window>Show Movie Properties (command J). In the properties window that opens, click on “Video Track” in the properties list, and then below that, click the “Visual Settings” tab. At the very bottom you’ll see a “Transparency” setting. Change that setting to “Composition”. Save the Quicktime. Note: This seems to disable the internet streaming Fast Start, so encourage clients to download instead of stream. Also note: I’ve noticed strange pixelation and jagged egdes that appear if I’ve downressed using any of Quicktimes preset dimensions options when exporting, so I always set the dimensions to “Custom” and manually type in what I want.
This method doesn’t necessarily solve the ever-mysterious gamma issue, but it seems to satisfy clients.
Best of luck.