That would depend on the type of insurance you have. We have ppo so no referrals are needed. Because of that I tend to go straight to the specialist. We did mention my son's joint pain to the GI and he said it was common with CD. I wanted more info than that since my son's knees would ache most often during aCD flare but his back ached constantly.
I had also mentioned to his GP in passing and he said it was probably just the Crohns.
At the first rheumatologist appt she did xrays, blood labs, and physical tests and said he had JSpA related to IBD. It is the juvenile form of spondylarthropathy and it requires a biologic and you need to have MRIs/stays to ensure there isn't fusion every so often.
Sometimes it's good to get the specialist of the issues to have a look.