Me and my husband got married before medical school. I think most of the above advice has mostly been solid (her cyclical time availability, cook for her/make sure she eats, don't begrudge leaving her alone to study, etc). I don't think this has been addressed yet, but I sorta skimmed the second half of the comments so could have missed it.
If you haven't heard yet, depression is a huge deal in medical school. One in four medical students will become depressed. And an entire class's worth in the US will commit suicide every year. I suppose I fall into those statistics as well as I became depressed in second year and had to be on SSRI's (anti-depressants) for about 8 months. I know when this happened to me I stopped responding to my friends' texts, stopped going out. I think everybody assumed I was just studying for Step 1 (a board exam) and none of my friends reached out other than the occasional text. None of my med school friends really pursued making sure I was ok though. I don't blame them, we are all in the same boat of barely handling our own ****. I didn't catch if you said you two would be living together, but because my husband saw me every (mostly) day, he was really the one that ended up encouraging me to seek help. I don't know what I would've done if I had been living alone and had nobody to really be vulnerable with and admit this to. It's easy to hide depression. I'm saying all of this because I just hope that you not only help her though the highs/lows, but also if the time comes that she needs help, that you can help her know that there is strength in seeking help and there's nothing to be ashamed of. As medical students we have a mentality that we have to be the best and fix our own problems and not ask for help so we can be perceived as capable. This is a dangerous combination when it comes to the pressures that medical school can have on mental health.
Hopefully your gf doesn't experience this and you both have a lovely time full of unicorns and rainbows. (ha!) It will definitely be a memorable time in both of your lives, but there will be some fun times for sure. And it will pass. Good luck to you both!