Air carrier Captain reported both GPS systems failed while in ZKC airspace but then returned to normal after some time.

Narrative

After leveling off at FL370; just south of MCI we received a scratchpad message stating L and R GPS invalid. Verifying on position reference page both GPS signals were not showing any data. I promptly contacted Kansas City Center and asked them if they knew of any GPS outages or jamming. They replied that they did not know of any and contacted a nearby flight to query their GPS status. The other aircraft was reporting valid GPS signals. I then referenced the QRH for ADS-B Out failure as well as GPS failure in QRH. GPS fail had us monitor the ANP value versus GPS RNP. The ANP value never exceeded more than 0.20 where RNP requirement was showing 2.00. During this time the FO continued flying the airplane and took over radio communications with ATC so I could contact Dispatch and Maintenance on our secondary radio. I noted to Maintenance that we had dual GPS invalid indications; dual transponder fail indications; but no lights on overhead Inertial Reference System (IRS) panel. We could not determine what could have caused the failure of both GPS units. I coordinated with Dispatch to descend to a lower altitude before ending our conversation. I then resumed full pilot monitoring duties resuming radio communication with ATC. We obtained a clearance and started a descent to FL270; upon descending below FL330 my FO pointed out that the ANP value improved greatly showing an ANP value of 0.02. I checked our GPS position reference and both GPS were showing a valid signal as well as both transponders resuming normal operation. We continued the flight at FL270 and landed at JFK with no further issues. An Aircraft Maintenance Logbook (AML) entry was logged for this event