All GLi routers come with OpenWrt preinstalled. The GLi GUI comes installed on top of this by default. To get to OpenWrt use the "Advanced" link at the top right of the GLi GUI.
The version of the GLi GUI may vary with device and inventory, but there is now a V2.25 that you should upgrade the device to. The underlying OpenWrt is their compiled version of 15.05 and is supported from thier package repository.
CC 15.05 was released before all the devices were mainstream, and you will not get a "released" version of this firmware from OpenWrt. I think you can get trunk with no Luci. You will be able to get released LEDE versions (with Luci) for GLi in the next few weeks