With Gem-tac you can't soak the garment for very long because the glue does soften up. I actually purposely removed all the stones from an old dress that I couldn't sell due to stains and other damage. I soaked the dress in warm water in the bathtub for hours and picked off the stones one by one. I had to do this a few times -- the glue gets softer with hotter water.
On the other hand, I've also machine-washed, on a short cycle, in cold water, a dress stoned with Gem-tac and I only lost a few stones. I washed the dress zipped up in a pillow case, so I was able to recover the stones and re-glue them.
A lot of how the stones stick is related to how they are applied. Some people don't use enough glue and so the stones pop off. Ideally the glue dot should be big enough so that when you push the stone into it, a small ring of glue oozes out arond the stone, like a little cup or rim set.