There are plenty of XC trails to ride in the Plenty Gorge area. None of it is built by MTB clubs, or ParksVic or other official interests, hence the aforementioned 'not legal' label. But it's more a lack of officialdom than illegality - bicycle riding is permitted in the park, so you're not doing anything wrong if you're riding in there.
There are two section of trails. One referred to as southside, which starts out from the end of Kariboo Gv, and the other unsurprisingly known as northside which can be accessed by crossing the Plenty River at a particular point on the southside loop, or from various park entry points off streets who's names escape me at the moment.
Regardless, it's too hard to describe the trail network here - your best bet is looking in the
Melbourne Social Rides thread here or a similar one on MTBR (in the Aus/NZ section) for a scheduled ride out there and just tag along.
In the past, various groups of youngsters have built some sizeable free-ride-esque jumps/ramps etc in the area. They were so poorly built I would hesitate to stand on them let alone actually riding on them, and often incorporate cutting down trees or nailing into growing timber. These sorts of constructions are what will draw the ire of ParksVic (and is most definitely illegal), and likely make it difficult for the rest of us to ride out there. The smaller stuff in the video above (it's in the southside section) is also pretty dodgy in places, but thankfully mostly constructed from just dead/fallen wood and fun nonetheless. Be sure to suss out the landing area of the jumps before you hit them ... but you probably already know that.
P.S. if you do drive there, avoid parking in Kariboo Gv. I've seen shattered glass there on several occasions.
