I'm not a doctor, so please consult your family practitioner- websites are not the place to make decisions concerning your baby. Seriously. Several of my relatives are nurses and hospital staff, so I will tell you something I know, but since I'm not an expert, (and nobody hear can diagnose your baby, obviously), ask your physician and he/she will tell you what you need to know.
First, of all, celiac disease and gluten intolerance (not an allergy!) are two different disorders. Barley is a grain, like wheat, oats, or rice. There is nothing unusual about it- it is long favorite in America and other countries. Our dependence on processed foods and a handful of food items has allowed us to forget that real food is natural, not stuff that comes packaged and ready to microwave. For some reason, people think foods like barley, greens, chick peas, or whatever are suspicious, because they are not common or no longer common at dinnertime in the US. Barley water, in fact, continues to be a popular drink, flavored with fruit juice, in England even today.
If your baby has a sensitivity to oats or rice, then surely, she may have a similar reaction to barley. Barley is a distinct grain, but as a cereal grain, it's proteins are similar to those proteins in other cereal grains like oats and rice.
Many different things can cause digestive problems, not necessarily the cereal. It could have been prepared incorrectly at home, someone could have given the baby a food and you were unaware of it (father, other children?). And 6 months may be a bit young for cereal, but it does depend on the development of the child.
Many people claim to be 'allergic,' but a sensitivity or intolerance is not an allergy- allergy means a reaction in the immune system- a person can stop breathing, have skin rashes, swelling, stomach pains, etc. Only your doctor can test and find out.
The people answering your question are not doctors or nurses, they have never met you or examined your child, and they don't seem to know what they are talking about, in some cases. Gluten allergy is the 'new thing'- everyone claims to have it. Gluten is what makes bread 'bready.' People have eaten bread and oatmeal their whole lives, get one stomach ache after watching Dr Oz or the Doctors, then they self-diagnose that they have a 'gluten allergy.' Just ask your doctor, and forget these people who don't know what they are talking about.
Hope your baby feels better soon.
http://www.mayoclinic.com/health/wheat-allergy/DS01002/ME...
http://www.foodreactions.org/gluten/