I tend to think that tales of weight gain from artificial sweeteners are psychosomatic. In other words: you could be imagining things. I've used probably every brand of sugar substitutes out there: aspartame, stevia, splenda (sucralose), sweet'n' low, truvia, equal, lo han, xylitol, erythritol, etc.
Many different versions of them: as liquid or in in powder from, including bulking agents like sugar or maltodextrin.
Sum total of my experience: some of them will raise your BG somewhat (but not greatly) and yes, overreliance on powder sugar substitutes can somewhat increase your BMI, because of the maltodextrin. However, most natural sugar subs with zero glycemic loads (stevia, liquid splenda, etc.) will not have that effect. In fact, stevia is slightly hypogylcemic. You might think you're gaining weight. But that's because of something else in your diet and metabolism, not because you haven't eliminated sugar substitutes.
Those with normal metabolism do not gain weight using Splenda or Stevia. The same is largely true with Aspartame, used in diet sodas and sugar-free jellos.
The Pavlovian explanation that eating something sweet (whether or not it's actually glycemic) will secrete insulin, and result in weight gain doesn't hold true for most people. At least for those whose metabolism isn't deranged. Dr. Bernstein tells stories of his patients relying on sugar substitutes with no weight problems -- and these are people with prior weight problems. Frank Hu also presented a metastudy which exonerated diet sodas from causing obesity.