The GFCF diet made no difference for our son. He was diagnosed at 23 months and by 25 months was in early intervention. He was also diagnosed with severe ADHD and it was recommended that he be put on ritalin at the age of 2 (13 years ago and it didn't happen). We finally put him on meds at the age of 4 1/2 and he was finally able to make eye contact and learn in school. We have tried several different things for him over the years. He had TSS, OT...still receives speech and has learning support in school. He is able to attend regular classes with typical peers. We did have him tested for silent seizures and celiac disease...it's been said symptoms of both resemble autistic tendencies. We also did the allergy testing to find he's not allergic to anything. Does your hospital or childrens hospital have a child development unit (CDU)? A developmental pediatrician will be key...they can point you in the right directions. Our son also attends a socialization program 2x/week outside of school. It's for older kids, but something to keep in mind for your daughter. Ours is called Wesley Spectrum Services - the program is called Wonder Kids. You can google it and see if you have something similar in your area. I've seen kids as young as 5 and it goes through the age of 19, all in age appropriate groups.
When we received the diagnosis, we went full steam ahead. He was completely non verbal with lots of self stims and no ability for pretend play and no social skills. In the last year, he has joined the junior high wrestling and track and field teams...where his teammates cheer for him (I almost cry just remembering it) and he's begun approaching people in social situations and initiating coversation. Our son also sometimes uses pen and paper to have tough conversations with us. He'll either write answers to questions down or draw pictures to get his point across.
I know this is a lot to take in, but with 13 years behind us we have found plenty that has worked for us. It will take time, but you will find what helps and doesn't help your daughter.
Good luck to you all!