if you go to the original secret spot from gmod, there should be a chair in there. You'll see a white square and then you can use the noclip gun to go through there.

You can also ask other people to help you out

When you grab the chair you will be teleported to the backrooms

how to you spawn the backrooms entities?

If you escape the backrooms, an entity spawns near spawn. It can only be done once per session. Currently the entity cannot be spawned by the means of the toolgun