You could try launching it from the terminal (the default terminal application in PureOS is called “Tilix”). Depending on whether you installed it via Flatpak or via the Purism repositories, the command to run it will be different.
You can check how you installed it by opening “Software” and searching for GIMP. I’ve attached a screenshot of my results. You can see the first result is from Fedora, because I’m using Fedora, not PureOS, but the second, which is the one I have installed, is from Flathub.
If you’ve installed from the normal repositories, you should be open to open Tilix, just type
gimp into the command line, and hit enter.
If you’ve installed it from Flathub, then open Tilix, type
flatpak run org.gimp.GIMP, and hit enter.
This should print some info, including possible error messages.