I believe there is some licensing around current and recent past WRC cars and the game franchise with the same game, so cars like Ford Fiesta, Hyundai i20, Citroen C3, Toyota Yaris or the previous VW Polo and so can't have rally versions on the game nor the "WRC" naming can be used.
So they used the naming "Group B", altough the Gr. B concept was completely different, as they were based on street legal cars of the same specs (to have a Peugeot 205 group B, Peugeot had to build 200 street legal 4wd, mid engine, 1.8 litre turbocharged 205 model), and the cars where super lightweight (to put in perspective, the minimum weight was related with engine displacement and cars like the Peugeot 205 or Lancia Delta S4 had a minimum weight of 890kg to about 450hp and Audi Quattro or MG Metro had a minimum weight of 960 kg to excess of 500hp on the Audi), most of the Gr. B in GT Sport are quite heavy. Except the Audi Quattro, the weight of the cars vary from 1180kg to 1380kg. So, right on point on HP figures, wrong on other aspects.
Another example is engine placement, there where many Gr. B cars, basically every low volume production versions (less 2500 units/year for version, 25000 units/year the whole model (like 2500 Evos/year, 25000 Lancer/year), but the true successful ones where mid engined, space frame "rockets", like Peugeot 205, Lancia Delta S4, Lancia 037, Ford RS200, only the Audi Sport Quattro was front engined until S1 version, only S2 was mid engined.