Gorilla glue is actually kind of a funky material to work with, as I thought the same thing. I 99.9% of the time get it on the front glass and 99.9% of the time get it off without a visible trace. I've come to find out that if gorilla glue is applied with no pressure it will not hold well at all. If you have the weight of the wood/rock on top of the glue you have enough pressure for it to cure very very strong! But, if you just get a drop, a small puddle, or a smear on something you do not want it on it suprisingly comes off very easy with a razor blade.
As far as curing good with a little bit of water, that is 100% true. If you are gluing non-porous materials such as glass or slate it is a good idea to moisten these surfaces before applying the gg. I have even used this stuff submerged, I would only recomend it as a last resort, but it worked flawlessly. I added a piece of wood to one of my paludariums one night and it was cured the next morning and it was VERY strong. The only down fall is it created a film on the top of the water. Which is not a real big deal either, it comes up all in one piece like you've laid a piece of serran wrap on top of the water. For those of you who like silicone, I strongly suggest you try gorilla glue! I guarantee you will not go back. It seems like it is very expensive but if you actually break it down it costs less than using silicone.