What she's doing is completely age appropriate. Everyone wakes several times during the night. As adults we've just gotten used to it and generally role over and go back to sleep without even fully realize we've woken up.
When we first moved our son to his big boy bed and he woke in the middle of the night he would slide out of bed and wonder into our room. It was actually adorable because it seems like his eyes weren't even open and it was clear that he was not awake. He still gets out of bed in the middle of the night, but before coming into our room he now has to shut the door to his bedroom, shut the door to the bathroom and shut the door to our room. It stinks, because now I have to get out of bed and find him!
My husband and I have agreed that this is just part of being a little kid. It's a phase that he'll grow out of. Thankfully we have a big enough bedroom for a king-size bed, so we just put him between us and go back to sleep. With our older son we found that trying to change the middle of the night waking was simply an exercise in futility so we just accept it and go back to sleep.
I recently went back to work, so I suspect part of the reason our son is doing this every night rather than 2 or 3 times a week like he did over the summer, is because he's adjusting to the new routine and needs that extra comfort of sleeping with Mommy and Daddy.
Personally, I am not a fan of putting up a gate and forcing the child to be alone in the middle of the night. I believe that children need us 24/7, and sometimes they need hugs and reassurance, even in the middle of the night.
Put her back in bed, let her sleep on a mattress in your room, bring her in to bed with you ... whatever you are ok with, just make sure you do it with love. She really will outgrow this. Right now, she needs to feel safe and secure.