gpa

No one can say for sure why you will get cut during recruitment. Sometimes the girl with the lower GPA, or fewer activities, or whatever will get invited back and you won't. It is best not to dwell one why you were cut and from where, but to focus on who you have been invited back to and what that chapter has to offer and what you have to offer it!
Yes, chapters have a "GPA Requirement" and you see it listed in the pamphlet given out at orientation; however, some chapters don't adhere to it as strictly as others. Sometimes chapters need to use it to cut out girls.
An example with some fake numbers, we have 1000 PNMs. The girls we want to cut are easy; the ones who were rude, the ones who are SERIOUSLY weird. So that eliminates 50 girls. We have to cut to 500 girls. So let's go back and cut the girls with a bad rep, had a rude post on Facebook, seemed like she might be too quiet/wild for us. Ok, another 50. Now we're at 900, every girl left is worthy of an invitation. But we're still required to cut 500. We have to invite back all of their legacies (many sororities have this rule for the first round) and there are 50 legacies going through. 850! We have to make some cuts! Anyone below our 2.9 requirement is gone. Now we are at 650. That's our 500, plus a good 150 flex list in case anyone drops out/doesn't want to be invited back. GPA is easy and an objective thing we can cut on as opposed to subjective things like if a PNM is super well-spoken or not or witty or whatever.

They don't include winter session grades in gpa for rush now. So if my fall grades are just below the cutoff will my winter session grades not even be considered even if they meet the sororities requirement?

They have already said this, I read it on the UD Panhellenic website. You have until February 8th to register after it opens and winter grades are not being calculated in unlike last year when they were. I didn't hear this as a rumor I read it on the official website for UD.

I wouldn't worry about it, I think like 2 sororities cut below 3.0. I rushed with a pretty low gpa and had a lot of sororities back and it turned out perfectly :)