There’s heap and heaps on threads on here discussing where to ride in NZ, I’ve contributed to them plenty.
For finding GPX files…. I dunno, I never used them. Most of the backcountry trails require so little navigational skills it wasn’t ever really necessary. If you know where the trail is and have a map then there’s often little to go wrong. Normally it’ll be “hike up a trail for 3 hours, turn around and ride back down” or “go on this trail, after an hour, follow the sign for XX hut”
Rude rock etc. are all signposted.
Most other backcountry type trails are self-explanatory.
Get a mapping app – the NZ topomap data is free to use so there’s lots of apps giving you access to (and ability to cache) the map tiles, legit and for free.
http://www.topomap.co.nz/
I still have my (Android) map app installed “New Zealand Topo Maps Free”. Can’t remember what the MRs used on iOS.
HTH