Unlikely for a Gripen to be in Area 51 just for some disinfo campaign.
The US has been testing "a few" prototypes for NGAD. They may be in competition and evaluation. If they work out ADVENT, that could extend the range by some margin without other methods of extending range - size, lift, and fuel capacity.
The Chinese one photographed also seem to be a flying wing configuration with no major moving surfaces. The graphics for "placeholders" in the Zhuhai airshow also seem to indicate flying wing. Basically "6th gen" is 5th gen minus control surfaces and in the US case, ADVENT. China has its variable cycle engine in development but unlikely to be as ready as the American one. Program major milestones probably 10 years behind the US? but everything trying to be done within a shorter timespan if the materials and manufacturing techniques are not obstacles anymore or mastered at a faster pace owing to number of people and teams working on those projects. Let's face it, both of them are working on blank cheque program budgets.