Hi, D. - I feel your pain as a mother, especially w/ the early wake-ups w/ your Gabby. My older daughter went thru ALL kinds of waking up moments between 12-24 months, some nights were just downright horrible.
Your daughter COULD be experiencing teething pain, since 15 mos. is a point at which toddlers have a MAJOR growth spurt, which affects eating and sleeping for the most part. If she wakes up lately around 4 a.m., and you let her cry it out, I'd say just let her do that. If she keeps getting used to your coming into her room and rubbing her back to self-soothe to go back to sleep, she'll expect it ALL the time...and the older she gets and you keep doing that, it'll get worse as she hits 18 mos., 2 yrs, etc...
As much as it's painstakingly difficult to hear her cry on the monitor, just turn the volume down a little bit, which might help. Sometimes, no matter WHAT we do as mothers, our kids just aren't happy, and they just have to live with their discomfort from time to time. As long as they're fed, dry, bathed, have appropriate medications to help them, and they're safe in their rooms, that's all you can do. Parenting is NO easy task no matter WHAT other people try to tell you, and you have to do what's right for YOUR family. However, when our kids are fussy and just unhappy, often the best thing you can do is nothing, 'cuz they'll learn that they're making mountains out of molehills without US losing our cool in front of them.
Before you know it, Gabby will be 2 yrs old and over this hump and on to something else, so like all funks that she'll go through, just ride this out w/ all the patience that you need to find so that you don't go crazy worrying about it too much. Trust me, Gabby KNOWS that you love her, and by loving her, you set boundaries for her. When she's a little older and still might pull the waking up stunts, balance out between helping her while showing her who's boss. Kids are slick, too, so be careful that she doesn't have u wrapped around her finger, either. What worked for me was I after 10 mins. of my older daughter's crying fits, I'd bring her into our room, have her lay on top of me, rock her back and forth, and rub her back while I sang "twinkle, twinkle little star" very quietly. After a few minutes from when I finished, I'd bring her back into her room, put her cd player back on, and just put her in her crib, and closed the door. Most of the time, she'd fall asleep on her own in no time. Other nights, she'd carry on 'til she wore herself OUT from crying. I'd cry, too, when she'd do that, but I also realized that it was for her own good to know that night time meant sleeping for EVERYONE in the house.
My older daughter is now almost 3, and now that we have a newborn, we made sure that she (the older one) was put in her new room w/ a new bed (just mattress on the floor) before the baby was even born. That way, she wouldn't have had to transition through ALL of those changes at once. She does really well, now, pretty much sleeps 10 hrs straight most nights.
Hang tough, this, too, shall pass...good luck, God bless!