what is the boost threshold on a bone stock TGP?

What do you mean? In the PCM or on the gauge?

what rpm does boost start at

Depends on ambient air density, age of turbo, if there is a filter on the turbo inlet or stock, what cam or age of cam, condition of torque converter (locking or not), etc.

There is no plain english every car is the same when the turbo begins to wake-up.