It isn't, they are called the G because in the 1990's GM standardised the model naming for all markets.
Vauxhall had sold the Opel Kadett 'D' as the Mk1 Astra and the Kadette 'E' as the Mk2 Astra. When the naming was standardised, the new Astra (Mk3) would have been the Kadett 'F', so they kept the designation and it became the Astra 'F', the Mk4 was the 'G', etc.
Yes they carried on the designation on from the Opels,they very nearly dropped the Vx name altogether at one point,
like a lot of dealers in the 70's & 80's we were selling Opels and Vauxhalls side by side.
Thinking back to working In the dealers at the time though,i don't recall the Mk1,Mk2 or early Mk3 Astra's being referred to as anything other than an Astra Mk 1,2 or 3,
however once the late (1998) Mk 3 (F) came out and they started putting the model in the vin (W0L0TFF) rather than using the old W0L0000 vin as in the earlier cars, we dropped calling them by mark and from then on they always got referred to by letter.