I would try cleaning the dial drive before doing anything drastic. It might be incredibly dirty or corroded, etc. from disuse, preventing the radio from tuning above a certain point (in your case 98 MHz). I have a Zenith C845 that would not receive FM or AM above about 105.7 and 1460, respectively, when I first got the set. I cleaned the bandswitch and tuning mechanism....presto, the thing now works perfectly (and no more noise when I turn the bandswitch; the thing was very noisy at first). No dial restringing or any other inside-the-box work necessary. I like simple repairs such as this.
As Chad mentioned, it could be also that your receiver only tunes to 98 or 100 MHz by design; the latter was (and probably still is) the top of the European FM broadcast band. Is your set's FM dial scale calibrated only to 100 MHz or so? If so, there's the explanation.
Good luck. The Telefunken sets, so I am told (I'm not familiar with the Gavotte 9 or any of their other receivers), were excellent radios. The cabinets, of these and many other German multiband radios of '50s-'60s vintage, were fine furniture pieces in their own right as well.
Get yours working as it should and you will have a fine set. You could even use this as the center of a reasonably good hi-fi system, as many of these radios were designed to be used with stereo phonographs and might even receive stereo FM, not to mention having outputs for external speakers. Just hook up a turntable or CD player to the audio input on the rear apron of the chassis (there should be a five- or six-pin DIN socket [Europe's answer to our RCA plug] back there; most German multiband sets had them) and you should be in business.