GDScript threading example with an asynchronous "blocking" OS.execute that doesn't block the main thread.

thread_execute.gd

extends Node

signal thread_finished(t)

var t

func _ready():
    t = Thread.new()
    connect("thread_finished", self, "_on_thread_finished")
    var params = {
        "thread": t
        # whatever you want in here
    }
    t.start(self, "_thread_method", params)

func _thread_method(p_params):
    # do stuff with p_params in a thread.
    # For example, a synchronous OS.execute that DOESN'T block the main thread.
    var output = []
    OS.execute(p_params.command, p_params.args, true, output)
    emit_signal("thread_finished", p_params)

func _on_thread_finished(p_params):
    if p_params.has("thread"):
        p_params.thread.wait_to_finish() # only synchronously execute the signal callback code
        # ^ note that you need this 'wait_to_finish' call or else the thread will never be rejoined with the main thread.
        # You'll end up with Object instances still active at exit if you don't call it (gives you warnings at exit).

Noticed I'd need to change the var t into a member variable so it wouldn't be de-allocated once _ready() completes (it's a Reference variable). Whoops.