The G stands for Generation.
The Elitebook 850 G1 for example, has the Intel 4th gen core processor series.
The G2, has the Intel 5th gen core processor series.
The G3 has the Intel 6th gen core processor series.
The G4 has the Intel 7th gen core processor series.
...and so on.
If you want to delve into the full specs for the models you are interested in, google Elitebook 840 or 850 G# quickspecs.
Here is the link to the 840 G1 quickspecs for example... The 4 in 840 stands for the size display (14").
https://noteboox.de/mediafiles/Sonstiges/HP-EliteBook-840-G1-QuickSpecs.pdf
You can get an 830, 840, 850...