We are going through this right now!
Monday, we took the front of the crib off and converted it to a toddler bed. Over the weekend, he started crib jumping.
We followed the 'no-emotion, silent return' back to bed. Why? If you talk with him, yell at him, beg with him, you are giving him attention which will only A) wake him up even more and B) reinforce his behaviors to continue to get out of bed in an attempt to get your attention. We also combined this with a 'star chart'. Every night he stays in bed, he gets a star. When he gets to 5 stars, he will get to start using his brand new Thomas bedding. The chart works WONDERFULLY! We ask him "What do you have to do to get a star" and he replies "Stay in bed".
Night one = 2 1/2 hours of getting out of bed. Until daddy came in and relieved me, that is. Once daddy took over he got out like 3 more times and called it quits. Hmmm.
Night two = Daddy was on duty. He got out one time because he has totally peed a gallon in his diaper and asked for a change. Daddy went in and changed him. No return trips.
Night three = If it ain't broke, don't try and fix it right? Daddy was on duty again. He didn't get out of bed once. Well...we don't count the time at 12:30 when he fell out of bed and I went in to help him get back in bed.
Moral of our story? For whatever reason, he responds well for daddy so daddy is in charge of this transition. Once it is no longer a novelty, I'll go back to helping with bedtime. On the bright side, it is totally cool to have my husband be the 'master' of a childcare technique; it is like their special little thing :)
A baby gate might work for you, but for our son well, he just jumped over it.
I am also expecting (end of December) and have two more weeks left until I go back to work as a teacher. Quite frankly, I am a little relieved that he initiated this change in sleeping arrangements NOW, when we have time to catch up on sleep and before the baby comes.
Good luck to you! It might be a few days or weeks of transition, but he'll get it as long as you are consistent!