I released Battery Monitor 0.5 for GNUstep. This version includes some bug fixes and most importantly it supports the ACPI information in the /sys filesystem available on the more recent Linux kernels (as compared to /proc).
I noticed that for some ACPI implementation information is written in both places, but for some it is only in /sys so BatteryMonitor wasn't able to find a battery.
Currently /proc is still available as a fall-back if no battery is found in /sys, but I plan to deprecate and remove it in the future.
Batmon is available as usual in GAP.