Hi, CBF, welcome.
. Your GF's drinking habits sound fairly similar to mine before I got sober--though I did find the quantity crept up gradually over about 10 years. But I did not generally get sloppy, I was capable of periods of sobriety (e.g. 2 pregnancies), etc.
Honestly, I had to come to the realization I was an alcoholic over a long period of questioning it and attempts to cut back. I don't think I could have just had someone tell me and have accepted it.
If you want to bring it up with her, I would NOT do it "lightly" or jokingly. I would sit down together at a time when you guys have lots of time to talk and I would ask her how she feels about her drinking. Does it worry her? Does she feel like she's overly preoccupied by it? What does she think having a problem looks like and how would she know if she was developing a problem?
I think you break down defensiveness by being curious about her, rather than telling her she has a problem. That being said (and after she has had a chance to share how she thinks and feels) you do have the right to bring up any ways in which you're negatively affected by her drinking. Is it affecting your finances? Is she harder to connect with when she's drinking? Does she avoid non drinking activities?