Once you have build a sled and removed/no-collided the gate, just run
also if you want to "correctly" use the gate at spawn without removing/no-colliding, run
(replace disable with enable or vise versa to enable/disable the pusher from pushing and close/enable the gate)
Also, if I'm correct, with this knowledge you could automate this with Wiremod.

Btw, the pusher occupies the area from the hazard strip on the floor towards and up to the starting line marked by the frame (or post, what ever it is called).

Edit: It appears, you have to learn even more stuff or even get a separate addon to control map entities/brushes with Wiremod.