I knew a bit about earlier versions of D&D from computer games, but was completely new to the newer version that BG3 is based on. I definitely found it a steep learning curve, but if you don't overcomplicate it I think it falls into that "easy to learn, hard to master" category and you should pretty quickly find yourself having a blast, but then want to start doing more research to better understand things like how to secure a better chance to hit, what the difference between attack rolls and saving throws is, and so on.
I think understanding the ruleset is something that will reward additional effort put in, and as Darth_Trethon says learning a few basics before starting can't hurt, but in general I'd recommend getting stuck in and learning as you go! There are plenty of good online D&D 5e resources, and though as folk have said, there are plenty of points of divergence from BG3, it's still mainly relevant so you can dip into them as and when you want to find out more about a specific topic.
