A weak Battery does require especial Charging.
1.Proper DISTILLED water fill of each cell.
2.Nice OVERNIGHT 1.5 to 2 Amp charge with a 3 step Charger.
(May be as long as 24-48 hrs.)
3.After Full Charge cycle turn on Headlights for a 10 second period to burn off
"Surface Charge". (Or wait 2 Hrs for Battery to equalize)
4.Measure resting battery BOTH with a DMM (for Voltage)and a Hydrometer
(For Specific Gravity).The Specific Gravity Measurement is the Ne Plus Ultra,
or True Determinant.
http://www.schumacherproducts.com/applications/?id=0007
Check ALL your grounds!(and connections "Clean,Shiny and Bright")
The Pre-Glow Relay has it's own ground to Chassis.
the Engine to Chassis strap Ground must also be good.
The fact that your GP relay is delivering 8 Volts tell you it's working.
"Somezing" is interfering with delivery of Full Voltage to the GP relay.
(Or delivery of Full Voltage from the GP relay to the Glow Plugs)
[AKA WIRING from GP relay to the Gps]