Hey - I'm 42, and have a 4 year old and a 20 month old, and am right there with ya!
We had our oldest sleeping with us until recently because when she was 7 months old she had a fibral seizure (her temperature spiked causing a seizure). I had a hard time putting her in her own room for a long time, and then we had #2. Recently (within the last month), we finally put them in their own "special" room. The oldest sleeps in her bed and the youngest sleeps in a crib. They've done remarkably well. It's taken the younger a bit longer to get used to it, but we're still doing pretty well.
We don't gate the room because we figure the 4 year old is old enough to not fall down the stairs, and can perhaps let us know if the younger one wakes up and/or come get us when/if she needs help.
We only really use the gate on rare occasions anymore...when she starts sleeping in a big girl bed, we'll probably use it at the top of the stairs. But the last time we really used the gate was to keep Megan out of the mudroom while the furnace repair guy was there.
I'd say whether or not to use the gate depends on how Nolan takes to being in his own bedroom for naps and/or bedtime.
One very important thing for us, particularly in getting our girls in their own beds without any hassle was having a consistant bedtime routine. Both of my girls can get "out of sorts" if the routine changes at all. Once we were finally ready to move them into the room, we gave them "advanced warning" (like on Thursday morning) that they'd be sleeping in their SPECIAL ROOM (on Friday night). BUT the routine was still the same. Went like a charm.
Good luck!