Ok, thinking hte same thing as Wendy - attach hooks to the wall, and attach gate. For around the side without a real bannister, you could even use a tie - loop through one portion of hte gate, nad up the stairs....
That way, you could just make the tie as long as needed to get to a working bannister slot. LO won't be able to get through, but you haven't permanently attached a gate, either.
Although, I do agree with really teaching her how to go up and down....cause that kind of set up won't last for long, and better she knows how to safely navigate stairs, then to slip through and hurt herself!