The 2007 Gazelle didn't have a 8ah battery but a 7,7 ah one (like the old Flyer Plus). Present Flyer have 10 ah battery or optionaly a 8ah one.
Gitane sells only a 8ah battery but it's exactly the same motor than the Kalkhoff.
Flyer and Gazelle (the later in a very goodlokking way imho) customize the Panasonic battery packaging, but others seems to uses standard Panasonic battery box. Flyer and Gazelle batteries are perhaps not compatible with other Panasonic equiped bikes.
I guess that Gitane 8ah and Kalkhof 10ah are interchangeable just like Flyer 8ah and Flyer 10ah are. What we need is a retailer selling Gitane and Kalkhoff to confirm this guess.