g36s only use v3 gearboxes. so yes, you will stay with v3.
A gear box is only as good as its builder. You can spend 160$ for a drop in and it can and will still blow up. A good tech can set it up for you to mitigate the odds it will blow up.
With the cost of the JG g36... just buy a second one and tune it to indoor spec and tune the other to outdoor spec.
There's no point in swapping gearboxes every time you switch play. You're going to strip the screw holes where they mount into the lower.
If you're dead set on getting a 2nd gearbox.
if you get any standard v3 you will need a g36 nozzle, the g36 safety, selector plate and motor cage. 95% of the drop ins are set up for AKs. If you do a 2nd gearbox, it's easier just to put together a whole assembled lower to swap in so there's not as much screwing/unscrewing involved. The motor cages on g36s are shiet.
For the cost of finding those and shipping them, you will be at half the cost of a new jg g36. After building up one or the other gearbox to do field or cqb, you'll be at the same cost of a 2nd gun.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.