Hello!

@luma: I got the same error message today forenoon in the application manager log.

@all: After I went home from work today evening I got the update-notification (the blinking exclamation mark beside the clock) for "batterygraph". I installed it and checked out if I could install "healthcheck", wow it was listed again! After I installed these packages I checked out again for updates and the error message in the log of the application manager with the GPG-error was gone.
I didn't change anything but now it seems that everything is back to normal. And I am happy again.

Hope this helps anybody.

Greetings from Germany