I am interested in applying to Ivies. I have good relationships with my teachers, good ECs and good testing scores. However, I am worried my GPA is too low (3.7-3.8). I go to a very challenging school, but is my GPA way too low for me to be considered?
Is that your weighted or unweighted GPA? If weighted it is very low for top schools. Without SAT/ACT scores, EC’s, class rank no one can chance you.
Short answer - it depends. Where does that GPA place you relative to your classmates rankwise?
It’s unweighted, weighted is much higher (above 4.0) bc I’m taking almost all honors classes. My school doesn’t publish class ranks 
You school may publish GPA’s by decile on the school profile. Even if they don’t your GC will have an idea of where you rank. Ask. A 3.7-3.8 in the top 5% will speak to grade deflation at your school. If a 3.7-3.8 is in the top 25%, then there will be the perception that a lot of students are more academically qualified. Trust that even without an official rank, AO’s will guesstimate, particularly if other students from your HS apply to the same college.
Rankings are virtually meaningless in most scenarios these days: there are too many variables and students are not compared to other students with the same course rigor. Your GPA is fine for Ivy–assuming high level work. If you get a 1400 SAT for example your grades mean less, if you get a 1550 your grades mean more. Your grades are just the first box…your test scores are crucial for validation and then whatever else you have that can separate you from the 1000s of 4.0s with 1500s.