When did GG start becoming an offensive term ?

[ReineD]

It's because women don't enjoy being referred to as "girls". We're not little girls. (GG = genetic girl)

Men don't like being referred to as "boys"?

I agree with Arbon, just refer to them as women. Or if you want to refer specifically to people who have XX chromosomes, "natal female" or "genetic female".

Edit

How is it possible to carry on a conversation if language is the real problem ?

It requires having an awareness of how others feel they have been belittled or repressed, and being sensitive. Many (although not all) women have been objecting to being called "girls" for years now. This comes from having had to claw out of the stark gender inequality of the 1950s.

If you unknowingly say something that another person objects to, just apologize for having been unaware and refrain from using the term again.