Hi Tobias Gerlach,
You need to make a change is snmpd configuration of GAIA system. As it is configured as non L3 device by default, Netdisco is skipping ARPNIP on device.
You need to edit "/etc/snmp/userDefinedSettings.conf" and add one line at the end of the file:
and restart snmpd service ("service snmpd restart"). then try to poll the device in nedisco.
Note: DO NOT EDIT FILE "/etc/snmp/snmpd.conf"
Hope this will solve the issue.