Great Stuff is for sealing around windows and doors.
Not for supporting shower pans.
You may need to pull the pan and reset it.
This time, someone needs to use a level when they are installing.
The pan may or may not need support underneath, depending on the pan,
In fact, the expanding foam may have lifted the pan.
If you do need something under the pan, something like a mortar mix may be better.
I will use piles of mortar, far enough apart, so that when the pan is dropped down, it can spread out and conform to the pan underneath.
I have shims place beforehand and the walls marked, so that when the pan is set into place, it's where I want it.