Did you try rolling the car into gear.
With the car off, one of your friends push the car SLOWLY while you apply pressure onto the lever to go into reverse. Not too much pressure.
There’s no Syncros on the Reverse gear so that removes that problem of the Syncros. I doubt a gear is damaged and the R32/Z32 trannys are pretty simple inside.
When was the last time the Reverse was used? Was there any hard driving between the last Reverse use and the time you noticed it doesn’t work?
You mentioned P-Mall with and reference to “we”
To most that will mean a showoff which involves hard pulls. Especially at Pmall with a GTR.