Well I'm not sure of what to do...
I might use the Picreator's Metal Decorroder, but I'm still afraid for the gold under...
Well, the gold won't be hurt...
If the iron/steel is badly corroded, chances are high that the gold will get loose though.
If the rust really has grown over the koftgari a lot, most likely it will also have crept under it extensively and already detached much of it. Thus, considering that badly pitted blades are also tough to clean and achieve a decent visual appearance afterwards, too, one might prefer to stabilize the rust first (and try to preserve the piece as relic by controlling the storage environment). Extensive tannic acid treatment?
Has anybody tried to reattach gold remnants to a thoroughly cleaned/polished blade? Probably a hell of a job for thin koftgari, I guess! Still, it might be worth to try to document any remaining koftgari/motifs in detail and then to decide whether to go for stabilization of the status quo (and accepting possible further damage to the blade or attached parts due to storage conditions) or long-term preservation/restoration or even redoing the koftgari in traditional style.
Kubur, I believe it would be good to post some pics of the very piece you contemplate to restore for getting more targeted responses!