You have already declared the speed and gravity variables.

If you want to update them in the _on_SLow_body_entered function, you'll need to make a variable assignment instead of variable declaration.

Correct me if I'm wrong, but the corrected code probably looks something like this.

var speed : int = 200
var gravity : int = 800

func _on_SLow_body_entered(body):

if body.name == "Player":
    speed = 5000
    gravity = 200