You get this from doing breach.
My teammates and I were doing breaches NONSTOP for 2 hours and 3 ppl got 7 gypsums, one got 4, and I got only 3.
I also had full luck gear on, even though I have no idea whether luck has any impact on gypsum whatsoever.
Why change? Because what sense does it give that ppl get it done and obviously want to leave and do something else, and you are stuck only with 3 pieces, having to do twice as much work just to get 2 gear score upgrade? Plus, breach rewards are trash, like, from all the chests I got zero upgrades, so there is no incentive for ppl to do breaches outside the mandatory 7 gypsums.