There were various complaints for the Gramps 5.1 and earlier flatpaks opening too many holes in the flatpak sandbox. Specifically, there were complaints about the flatpak having access to the entire home directory even though a system install of Gramps would have that access too. So the first Gramps 5.2 flatpak has restricted the sandbox down to Pictures, Documents, and Downloads.
The easiest way to open up access to other directories in the first Gramps 5.2 flatpak is to install flatseal from flathub. Flatseal gives you a graphical interface to change sandbox permissions of any flatpak app on your system.
Another way of opening up more directories is to use cli commands to manually adjust permissions. If you need help with this, then please use the flatseal app instead because it is a lot easier with fewer chances of mistakes.
I am testing a second Gramps 5.2 flatpak with access to more data directories. I will make a new post and tag Nick.