Edit: two hours after posting my answer, ComFreek detected it is the same product as in Tom's answer where it was named slightly different, and I didn't recognize it by the screenshot. For the additional details and screenshots, I leave my answer here nevertheless.
I don't use a GUI for Cron, but for beginners, Gnome-Schedule looks great and fitting all your requirements:
- Preferably Gnome: Yupp, that's already in the name
- Gratis: Yes.
- GUI based crontab editor: Yes.
- Ubuntu: Yes, that's what the Ubuntu Wiki (German) says :) It's in "universe".
You can find a closer description in an article on UbuntuGeek, which also includes some screenshots. Let me pick two:
Create a new task in Gnome Schedule (source: UbuntuGeek; click images for larger variants)
Another nice quick tutorial can be found at Dambalah, and its screenshots a.o. show how a list of existing jobs is presented:
List of scheduled jobs (source: Dambalah)
An article by Bart Bania shows another interesting screenshot:
New Job Details (source: Bart Bania)
Conclusion: When in need for a Cron GUI on Gnome, Gnome Schedule should fit your every needs. The menu bar shows I didn't even cover all features here (e.g. templates) – but as I covered all you've been after, that's for you to find out. Enjoy!