Here is a fluke meter + voltage source to test Gigohms with simple math

GAN diode LED  p/n C503B-GAS-CB0F0791,  Datasheet

Resistance

Good lead in video

 

high value test resistor,

1 gig resistor  pn SM104FE-1000M-ND

10 gig

 

Capacitor

capacitance

Microcurrent