What is the difference between the GC3 and the new GC3e? I believe only the GC3e supports new encrypted sensors and alarm zones. Is that correct? Anything else?
Correct, in order to use the encrypted E-Series sensors, the E-Series panels are required. Compatibility with the encrypted e-series sensors would be the biggest difference aside from physical appearance of the panels over their non E-series counterparts. The user interface appears to be largely unchanged.
The GC3e does offer a slideshow mode when not in use.
More information can be found here.