Gibbon was originally coded on a server with PHP Notices suppressed, meaning it was once rife with such errors. We went through a big bug squash a year ago fixed as many of these as we could find. Some still remain, on pages we don’t code on much…like the one you have found.
I have fixed the one you have reported.
For production, it is recommended to turn all errors off from onscreen display. You can always look at the log to see them if you need. For testing, I would turn notices on, unless you are of a mind to report any notices you see to us ; )