I've only limited experience with German electronic terminoloy, but was under the impression that 'Leerlauf' applied to a circuit without load attached - ('open-circuit'). Most (voltage feedback) amplifier circuits are still 'closed loop' even when no load is attached.
Taking an audio amplifier as an example, I'd expect the 'open-circuit gain' to be the gain when no speaker is attached and the 'open-loop gain' to be that when the feedback was disabled. The 'open-circuit gain' is thus equivalent to the 'closed-loop' gain, and in a good amplifier should be as close as possible to the gain under load (this implies an 'open-loop gain' which is as large as possible).
To show some typical values - consider a unity gain buffer constructed with the ubiquitous 741 op-amp:
'open-circuit gain' ~ 'closed-loop gain' = 1.
'open-loop gain' (A) ~ 10^6.
'closed-circuit gain' ~ 'open-circuit gain' x R_load/(R_load + R_output/A) ~ 1
where R_output ~ 50 ohm
Thats quite some difference!
'open loop gain' and 'open-circuit gain' are not equivalent in English, and are thus unlikely to translate to the same German term. I suspect that 'Leerlaufverstärkung' is the correct translation of 'open-circuit gain'. I don't know the translation of 'open-loop gain', although one specialisation (at least for op-amps - or other circuits based on a differential amplifier) would be 'Differenzverstärkung'Dictionary: differential gain