type /pg in chat to see all hotkeys

If I understand it correctly, it's shift+p, but it doesn't work. I am using PG now for years and never had this issue

That's for the mode, there's also the instant one on shift+rightclick.

But yeah I also see it not working, so many issues with the input changes from SE v206 and nothing documented xD Issues only from reading control IDs though, so you can work around by rebinding it to a specific key combo if you really want to use the mode.
You can rebind in the config menu (chat then F2, then top-left shows a "Mod settings" button), or in the config file (path in the description).

Found one solution, I've removed the shift+p keybind in the main game and restarted the game, after that the paint gun worked again. Similar thing happened to the terrain tool too.