You can control the command line prompt either with the command PROMPT or with the variable PROMPT. For an example, the following two commands have the same effect:
PROMPT $G
SET PROMPT=$G
Ultimately, however, it is the variable that determines the prompt's formatting, and when you are using the command, it merely sets the variable for you.
With that in mind, you could just assign a default value to the PROMPT variable using the SETX command:
SETX PROMPT "$G"
From now on, even after you reboot the system, every new1 CMD session's prompt will be formatted as $G by default, until you change the variable again or reset it (globally). You can globally reset a variable by assigning it an empty value like this:
SETX PROMPT ""
1The CMD session that issues the SETX is not affected.