Send signal to gpio when 8V power drops
Voltage divider:







simulate this circuit &ndash; Schematic created using CircuitLab

The actual resistor values do not really matter that much, as long as \$\frac{R1}{R2}\$ remains \$\frac{3}{5}\$. The 8V connection is connected to GND through \$R1+R2\$, so this shouldn't be too small; but if you select them too large, the pull on the GPIO might be too weak. R3 is optional, it just reduces the current flow further.

For this to work, the GPIO may not have a pull-up or pull-down configured.