For UAL, yes you can just walk up to the gate. Agent should issue a boarding jump seat pass, the Capt sorts it out--If there are no seats in back. If seats available, they'll just print you one with a seat.
But....you are still required to introduce yourself as a jumpseater.
No jeopardy, but you can also list ahead of time. Opinions vary as to the etiquette of not listing ahead of time. No worries it you don't.