It was possibly a fairly local thing.
As an analogy, when I started teaching on the northern edge of Sheffield in 1975, all the kids used 'non good ' and 'bit good' in almost every other sentence. 'Non good' meant "absolutely fantastic", whereas 'bit good' (always uttered in a disparaging tone) meant 'utterly pathetic'. Friends who taught up the road in Barnsley were familiar with the terms but those who taught in schools in central and southern Sheffield had never heard of them. They only seemed to stay in use for a year or two anyway.
So "gaffin in" might be an equally local term, only used within a very small area of London, such as a single borough.
However it's also possible that 'gaffin' might be related to the definition of 'gaff' offered by the OED as "Humbug, ‘stuff and nonsense’ ".