The design of wideband slow wave structure (SWS) for millimeter wave travelling wave tubes can be aided by the implementation of gap waveguide technology. Contactless metal walls can alleviate assembly challenges at frequencies above 70 GHz and potentially reduce ohmic losses. The increased design flexibility typical of this technology can lead to the integration of different functionalities into a monolithic design at the power coupling section. A W-band gap waveguide-based SWS is proposed for wideband travelling wave tube operation at the frequency range 85-100 GHz. Preliminary results of the complete SWS are presented.