According to Wiki, yellow number plates were compulsory on all vehicles manufactured from 1st January 1973; they had been available as an option since 1968.
From the Wiki 'Inspector George Gently' page: As the series begins, the death penalty is still in effect in Britain, and a trip to the gallows ends some early episodes (capital punishment for murder was abolished in 1964).
So it's just feasible that, 8 years on from that first series in 2007, GG is now in the late 60's, so maybe the yellow plate isn't an anachronism.