Flying wing configuration aircraft not able to track loiter, a constant crosstrack error

Hi,

I am trying to fly a flying wing aircraft, using catapult launcher. Takeoff is good and climbing. However after trying to loiter or even following a path. There is a constant cross track error. Any ideas what might be the reason. Error in GPS, or requires adjustment of L1 control. Adjusting L1 gain , still not working.

Log file link: Flight Review