The GI effect should be just directly correlated with how well large intestine microbiome can ferment these, IIUC? And I guess also with how much you need to have equivalent taste of 1 g sugar.

As in, if it tastes sweet but is not absorbed in the small intestine, so has "no calories", it will inevitably all pass on to the large intestine where it can be fermented.

As someone who absolutely hates the synthetic taste of aspartame etc. but has to stay on a low-FODMAP diet, I've just resigned to eating stuff with ordinary sugar and using sufficient moderation.