Drop a level from the edge of the flare and measure back to the hub flange. Marvic only make the 14" width rear (not the GT5 / GT5S 13") and they sit out to the edge of a GT5 / GT5S flare.
Marvic have technical documentation on their site with offsets.
Group 4 https://www.marvic.it/public/p...0-14x15_2016-eng.pdf
GTS 10" https://www.marvic.it/public/p...TS_8-10_x_15-eng.pdf
This thread may be of help, with photos of Marvic's 10" GTS rear wheel. https://pantera.infopop.cc/top...and-roin-information