It's a term I'm increasingly seeing used by people describing their experiences with 'trans' children, in articles like this one, where the author/parent is so thoroughly steeped in gendered assumptions that in her reality, girl = pink, princess, sassy, sparkles, and Disney movies, while boy = dump trucks, soccer balls, pockets full of rocks, and liking to eat. Therefore when her son preferred the things she associates with "girl", the logical conclusion was that he's really a girl, not a boy who prefers dress-up to dump trucks.
It's the end result of incredibly rigid expectations about gender, and the ultimate in gender conformity: "you don't conform to rigid expectations of what a boy should be like, therefore you are not one." 
And yet... This author, and others who subscribe to the ideology, use the term "gender non-conforming" and the idea of "shattering gender" as a synonym for 'being trans'. From the linked article: "Our rigid expectations around gender are being shattered by the upcoming generations" and "1 in 2 kids are at a risk of committing or attempting suicide if not supported with their gender nonconformity."
Now that "gender critical" seems to be catching on as the accepted term for those of us who recognise gender as a harmful construct, can we rescue 'gender non-conforming' from being co-opted to mean the opposite of what it actually means?
Surely it's people who are critical of gender who refuse to conform to it, who try not to push their kids to conform to it, who are working to 'shatter' it? Not those who believe in and promote gender as an intrinsic and edifying aspect of male and female identity? 'Gender non-conforming' has long been a descriptor of kids and adults who put two fingers up to the idea that 'being a girl/boy' means dressing or behaving in a certain way or having certain interests and not others. If I wasn't following this issue closely, I would hear people arguing the case for 'gender non-conformity' and that's the side of the argument I'd think I should be on, maybe not realising that it doesn't mean what it used to mean.
I know there are bigger fish to fry. But language is so important. Should we be making a point of using/claiming 'gender non-conforming' as representative of our position, and challenging its use by people who think it means that if the personality doesn't conform to the sex, the answer is to make the 'gender' conform to the personality?