I currently have Yosemite installed on my iMac - If I install the El Capitan GM will I loose all my installed programs and configuration or will it just do an upgrade?
Thanks in advance
You should be fine, but keep in mind, that is possible that apps may "break" in an OS update. If you update correctly, it is unlikely that you would lose any documents or data.
One exception might be iPhoto, as it was replaced with Photos. You might lose that.