entropygirl, it does indeed happen, sadly.
I fully understand why some treatments may not be offered, but liek you - to deny even that they are worth investigating is just sickening.
but yes, approach a gp with an autistic child who has gut issues and the answer you iwll get is 'yes, that is to be expected. next!' no curiousity as to why, no investigation at all.
the only way to deal with it is to do some research, find the people who are trying to help, and get on with it (privately and expensively)
I even offered to dd1's then team of paed, gp, dietician, to put her back on a gluten/casein diet (they wrre denying the benefits and writing them off as 'coincidence' - another word that crops up a lot in autism circles), so they could take a baseline, and then ee the benefits firsthand - in bowel health, in pain response, in language, in everythign really.
but they refused. not because it wouldn't be ethical (to put dd1 back through the misery), but because they wanted their nice cosy position of 'the parent believes it helps'
and yes, trying to avoid similar gut issues in dd2 (there tends to be family associated sensitivities) nearly triggered a child protection issue, as dd2 was being investigated for FTT, and already had gut issues (wonder why I wante to keep her gf/cf?) and yet I was told I was doing her more harm than good, and that it was all in my mind (dd1's imprvements etc)