That is the G is used to denote the ground pin, 3. The labling is also similar to the Arduino boards. As you can see each pin is neatly labeled against it.

The complete pin-outs of the STM32 Blue pill board is shown below. You might also be interested in the Reference manual and the Errata sheet of the Microcontroller as well which is also linked. If you want to know more on technical features of the Microcontroller then you can refer to its Datasheet. This is just an assumption and I have no source to back it up.

May be since the board is blue in colour and can give a boosted performance to your projects someone came up with this name in it just stayed. It also has two header pins which can be used to toggle the MCU boot mode between programming mode and operating mode, we will learn more about these later in this tutorial. The pin of the MCU are neatly pulled out and labelled as header pins. Because of this, the MCU can operate in deep sleep modes making it ideal for battery operated applications. These boards are extremely cheap compared to the official Arduino boards and also the hardware is open source. It looks very much similar to the Arduino Nano but it packs in quite a punch. However, as we start building more and dig deep we would soon realise that Arduino is not industry ready and its 8-bit CPU with a ridiculously slow clock, it does not give you enough juice for your projects. Arduino would have been the first board for many hobbyists including me and engineers out there when they started with electronics.

Stm32 blue pill schematic