Should I turn up the sides of the geotextile?
Geotextile is a mechanical protection. It is usually never bought of the right size: it is better to abund and make a lapel, which should never be fixed. Otherwise the waterproofing may be damaged, with risk of infiltrations and structural problems of the slab and in that case all the work has to be redone.
The panel does not fit completely. Can I fill the empty space with gravel?
Yes, it is a possible solution (viable also for sloping roof for drainage).
How is the irrigation system built?
It is build with perimetral tubes, ie drip irrigation system, laid on the geotextile on the panel.
I need to parcel out the garden roof (for example, a communal garden) with fences. How should I do?
With flower pots, brick walls or railings. It is important that the upper stratigraphy allow to anchor those elements on the substrate; otherwise it is required to install some anchorage before the installation.