Straight from the horse's mouth... De Gendt will focus (more) on GT from now on. Thinks he can win a GT... maybe not the TDF but he thinks Giro or Vuelta might be possible. He'll start training in the winter specifically with GT's in mind.
For those who speak Dutch, you can see a series of video interviews with him:
http://sporza.be/cm/sporza/wielrennen/Tour/120717_TDF_De_Gendt
UPDATE: In the Sporza talkshow during the TDF, Vive le Vélo, De Gendt again said he would ride the Vuelta for a stage win (not GC), because he doesn't want to go too deep since he wants to be good for the Worlds. Though if he is still close to a top 10 spot somewhere in the second week, he might change his mind and go all out for the GC.