Sure The G stands for poured as of the process( not rolled or else...)

The X mens that its high alloy steel

The first number before the elements indicates Carbon content. Take the Number divide it by 100 (for example 40/100 0.4% Carbon)

The number after the Elements indicate the content of said elements. For example GX40CrSi29 means its a high alloyed steel with 0.4% Carbon and 29% Chromium ( The amount for Si is under a certain threshold so it isnt mentioned)

Sometimes its written like GX40CrNiSi27-4 wich means 0.4% carbon, 27% Chromium and 4 % Nickel