What you see happens because you are only adding gravity when you jumped, so the gravity is not added when already on air. Also, move and slide is only being called inside the if where you jump... try something like this:
extends KinematicBody
var velocity = Vector3.ZERO
var gravity = 100
func _physics_process(delta):
if Input.is_action_ just_pressed("jump") and is_on_floor():
velocity.y = 10
velocity.y += gravity *delta
velocity = move_and_slide(velocity, Vector3.UP)
Note that i moved the last two lines outside the if, and also, added a check to see if the body is on floor before jumping, to avoid jumping in middle air. Also, you should update velocity with move_and_slide
so gravity doesnt sum up while on floor.
Edited as people in comments noted it was missing up direction