Being a slow gainer isn't a disaster.
we all know babies grow in spurts.
my dd was always kissing the bottom of the growth charts, at 3 years, she still is - she weighs just under 12kg (around 25lbs), and she's not too tall. but she's very developed in all other areas, and advanced in her speech and already writing her own name.
my cousin was the same, always tiny, bf until 3, and grew up to be a champion runner and is now studying pre-med.
just keep breastfeeding on demand, your baby will surely get everything she needs.