Almost certainly not. Autism is caused by developmental changes. This means that any gene therapy would have to start before the embryo development is even finished, i.e. in the mother’s womb. After that, it’s essentially too late. Think of it this way: Could gene therapy make an adult person taller or smaller? Despite the fact that height has a large genetic component the answer is obviously no, since skeletal growth stops after puberty.
> I believe it's now actually agreed upon that autism rates actually ARE increasing
You’re jumping the gun there. It’s plausible that there are factors that are causing an increase of autism rates (e.g. ageing parents) but so far there’s no positive evidence that this is indeed the case, or that diagnostic changes aren’t the sole reason (see https://en.wikipedia.org/wiki/Epidemiology_of_autism#Changes...).