GTO and 959 were both group b, iirc. group b wasnt just limited to gravel hence this sort of thing
They changed all the formulas around 1982/3 as when I was growing up we had the following:
Group 1 - Production based touring cars with fairly tight rules
Group 2 - Production based touring cars with less tight rules
Group 3 - Sort of died off replaced by Group 4
Group 4 - Sports cars which looked like the production version
Group 5 - Production based sports cars but highly modified
Group 6 - Prototype sports cars
These were the main categorise that covered track and rally cars into the early 80's.
The British Touring cars were Group 1, made up of things like Capri's, Rover's, RX7's, BMW's, Audi's. The European Touring cars also allowed Group 2, so things like Zakspeed Escorts, Camero's, CSL's, Skodas etc. The Group 2 cars were allowed a higher state of tune and also bigger and wider wheels and tyres with extended arches and some spoilers. For me, this was the best Touring car ever and surpassed the Super Tourers and RS500 era.
Rally cars were sort of Group 1, 2, 4 and included Escorts, Sumbeams, Chevette HSR's, Stratos, 911 etc.
Le Mans as sports cars were Group 4,5,6 and were also epic cars with things like the 935K3 and Lancia Montecarlo's standing out to me in the late 70's early 80's together with Group 6 cars like the 936, Alpine Renaults, Mirages.
In the early 80's it all changed and we got Group's A, B and C pretty much covering all of the above. For me this was the peak of motorsport as we had tech and performance starting to show itself, so they were incredibly quick and loud but also fun to watch as it was early stages of aero making the sort of impact we see today.