How to measure gigaohm resistors?
Many Fluke meters (e.g. 87,287) have a nanoSiemens conductivity range which will measure up to 100 GigaOhms - it needs to be manually ranged up from the ohms range. \$\mathrm{1 G \Omega  = 1 nS}\$, \$\mathrm{10 G \Omega  = 0.1 nS}\$. 

Alternatively, most DMMs have with a 10M input impedance (easily checked with a second meter), so a resistor with value R in series with the millivolt range will form an R+10M/10M voltage divider. So applying 10 volts through a 1 gigohm resistor will read around 99 millivolts. 
A close-enough approximation for high value resistors from a 10V supply would be resistance in gigohms = 100/millivolts.