I made a shelf between the Z-bracket between the gear Towers and the Z-bracket behind the baggage compartment. On this shelf I mounted on top the GSU-25 and the GAD-27 and below the GEA-24 and the FLARM (anti collision device mostly used in Europe). Here are some Pictures (not yet finished with the cabling thus it looks not organized).
I hope this helps. I cant yet say if my shelf is stable enough as the Manual calls out a very stable shelf for the GSU-25. But I am confident otherwise I will replace it by a much stronger one.
On the shelf everything is done with anchor nuts to be able to unmount a device more easily. I can reach all the connectors from below the instrument panel or by removing the big GDU-460. So I do not have to deal with nuts and washers once everything is installed. This installation also allows me to remove the baggage floor without having to unscrew something which is mounted to it. Thus it should be easy to check any cable from the panel towards the engine / firewall.