Basically, I’m a freshmen who just had to drop out two months in due to health reasons. I won’t be able to return until next fall.
is it gonna be hard to make new friends going back for the second time? Am I gonna end up isolated and alone? Is it gonna be weird that I already know where everything is? Or am I gonna be treated like anyone else who took a gap year?
I’m just kind of panicking right now
Nonsense. You’ll be treated the same. Just because you took a gap year doesn’t make you some alien.
No one’s going to care.
It won’t be weird that you know your way around. Many colleges have summer camps for younger people on their campuses and many of those campers later attend that college and know their way around.
You’re thinking about it too much. You might face some of the things you have gone through the first time but that should be easier to handle since you’ve already had the experience. Think of it as an advantage not a disadvantage 
If you were fine the first time around there is no reason it won’t be fine the second time around. Also, you will likely resume some of your old friendships.
If you are living in a freshman dorm, everyone will assume you are a freshman, which you are. They would never know you took a gap year unless you told them, and even if you did, they wouldn’t care
Unless OP maintains friendships with classmates who will then be sophomores, it’s possible it could come up without OP mentioning it. But I still agree that they won’t care.