I agree that having a GALS would be awesome, but it is way, way too risky for the environment. Non native species are a horrible thing to introduce to an ecosystem and I see snails as being a huge threat. If a slug or snail can crawl on something, then they will be tasting it. I could feel the scraping of the banana slug's radulas while holding them (feels like sandpaper!) and can see how they would obliterate a plant. I can only imagine the destruction to crops they would cause, especially after a few generations of snails are produced. This would potentially only take one snail, by the way. I am not sure if they are self fertilizing at all, but they can really store sperm for sure. That is how I got my little feeder snails.
About the banana slugs, they get HUGE. I found those guys in a sitka forest near the Oregon coast. I put them back after snagging the photos of them (oh man, I love slugs and think they and snails are adorable! Other gastropods, too!) and let me tell you, the slime does NOT come off. I had to wipe it on rocks and stuff. If you get it wet, it makes it worse. You have to wait until it dries and then rub it off. Snails on the other hand, don't produce slime like the slugs do. I will sometimes take my big snails out and let them roam around on a finger while I look at them with a loup and they don't seem to mind at all. My hands are perfectly dry and normal, even after they crawl all over them.
Funny thing about the banana slugs, raccoons will sometimes eat them and to combat the thick, defensive slime, they will quickly roll them around in dirt and then munch in on them. Slimy dirt covered spongy meat-tube, anyone?