These Griqualand overprints are quite troublesome!
It seems to me your 'G' is closest to the Scott type p - if so making it a Scott no. 74. However I agree with you, there is no good match with the illustrations in Scott:

It might very well be a fake overprint, can't tell. I presume most of mine are fake anyway. Then again - how accurate and detailed are the illustrations in Scott? I'm waiting for delivery of a more detailed St.Gibb catalog - I hope they cover Griqualand in more detail.
Presume some of our South African experts will chime in with more qualified opinions...