I don’t really understand the question, but I’ll give it a go! Whether or not you get GCB is dependent upon the condition of the vehicle upon return not the length of lease, which is irrelevant, other than you get more GCB after 4/5 years.
Motability allow ‘fair wear and tear’, so a car that has been on lease for 4 or 5 years will understandably show more signs of wear than one that is returned after 3 years and that would be acceptable.
The threshold for ‘fair wear and tear’ is very high and seems to include things that I would personally consider to be minor accident damage rather than wear and tear, so I really wouldn’t worry.