Ahh, have you checked to see if the gear selection cables are still attached??
Because everything you have described is of a car that is in neutral, the gear lever being moved, but the gear cables are disconnected, so the gears themselves don't change.
A/ but if it will go into gear while running without clutch I would guess clutch failure
B/ if it goes into gear with clutch but doesn't drive if would go for a diff failure
C/ if you can select gears whilst engine is running without pressing the clutch down
then its has failed between engine and gearbox.
Read them all, they all describe the same thing, Gearbox in Neutral, gear shift cables NOT manipulating the gear selector when the gear lever is moved.
See if I am right, when the engine is off, clutch in, as you move the gear lever you should be able to hear the clacking of the gear selector as it moves through the gear ranges . . . .I reckon it will eerily quiet
Also, in your opening post, you gave the answer yourself - "last night, it just felt like it kicked out of gear" (into neutral)