When you did the background how long did you allow the Great Stuff to cure before adding the silicone on top and then putting your soil to the silicone.
Gorilla Glue dries permanent and if you make mistakes it is not going to come off I believe. It is also almost impossible to sculpt it once dry but anything it touchs such as your hands, accents, glass, is not going to be easy to remove if accidents occur while applying it.
As for smell, the GE Silicone 1 product needs to air our for quite some time since the smell sticks around for a week but GE Silicone II usually has the smell mostly gone after about 4 days with good air circulation.
Great Stuff should be allowed to dry completely so if you put it on really thick it will take more than a few days to completely dry inside the deeper sections. I puncture holes to help with drying and allow for the still expanding areas to release anything trapped under the already cured top skin.
If attaching bricks (sounds heavy) a glue gun isn't a good idea since it separates from Plexi and glass fairly easily.
100 Percent silicone is good as would be the Gorilla Glue, or even a little bit of the Great Stuff can be used to attach the accent items.
Take your time, give things time to dry and air out, before moving on to the next process when applying anything that has to cure.