Hello Juevi,
At the end of December 2020 I purchased a 2015 F Sport here in Pennsylvania. I also read the reviews that mentioned the perceived difference between 2015 and 2016. I believe the reference is to the transmission. However, I do not believe that pertains to the GS 350. I think it is in reference to the eight cylinder version. As far as I can tell, the GS 350s with the six cylinder engines continued with the six speed transmission. That said, I sought a 2015 because I did not want the gigantic grille.
The car is terrific! Mine had 58,000 miles and was well maintained. It is a blast to drive.
So, as you search for your vehicle, just look for the six cylinder powertrain. Additionally, I think you'd enjoy the F Sport option.
Best of luck to you.
George