Well for me it's an EP because even though it reuses past systems it still larger then a GP. I think if it was a GP it would only add in the active Acting career and the world, the fame system/media skill and the world, or the fame system/media skill and a WFH/rabbithole acting career. I do not think we would get all 3 in a GP because one the Active acting career has a lot of assets in itself that would fill up the majority of a standard GPs assets.
I think part or the reason players are looking at it as a large GP (besides the world size) is because Fame itself. In past games be it done well or poor it was overall simple and a straight forward path. Fame in GF looks a lot more detailed and seems to give you more control over what kind of celebrity your sims can become. Also as mentioned Fame is something that not only effects sims who join the acting career or use the new editing tables but most of the game itself including all previous packs. You can become famous for your archeological discoveries, DJing at the nightclubs, or by becoming renowned art critic. And most likely as new hobbies and careers are added you can become famous from them as well.
Now it's because of the fact that Fame carries into all the other packs that make it worthy of being an EP as most features in EPs at least in ts4 have been design with the intent of working with and enhancing every pack. GTW has retail, GT has clubs and Seasons has weather. CL has no real new system and some would argue that Pets themselves enhance every pack.
Now GP on the other hand are more self contained and though detailed tend to not add features that really expand with each new pack. The only ones that do however are DO but unlike retail in GTW you can only sell food and drink, and PH and though character values have carried into other packs it doesn't feel like it's doing that much.
