Well I know in coeliac that the villi in your GI tract are flattened abnormally. I guess with plain GS it's just a food intolerance (like mine for dairy BEFORE IBS) without the physical abnormalities.
Getting diagnosed with Fibro is just a matter of finding a doc who knows something about it! WHich is WAY easier said than done. The usual diagnostic is looking at your history and doing a tender point test. They look for 18 tender points on your bod...some people say you need to have 11 to have Fibro but this is NOT true (it was an inclusion test for a research study never intended to be the definitive diagnostic).
Low immunity is pretty typical as Fibro bods are so out of whack...I know Ruchie and I both tae a load of stuff to boost our immune systems (getting a cold can be PAINFUL). Fatigue is the classic one, especially with non-resorative sleep. Back aches yup...there are typically ALOT of trigger points and tender spots in the back so this is ofte the first one people notice. All-over aches and crippling pain are NOT neccessary btw. IBS is another classic symptom as are intolerences to all kinds of stuff. Tension headaches or migraine are another "classic".
Basically, just talk to your GP and see what they say. If they don't know much about it, ask for a referral to a rheumatologist (they tend to know the most as it was often confused with rheumatoid arthritis).