Every high school has a different way of calculating grades, which makes it difficult to use GPA as a metric for college admissions. Some colleges provide aggregate GPA data for recently admitted students, but this is nowhere near as widely available as test score data.
Some colleges standardize GPA themselves through a recalculation process, which can provide a clearer point of comparison for students. Others focus on holistic review of student transcripts, with an emphasis of rigor of curriculum – that is, they primarily care about which courses your student took, how challenging those courses are relative to what was available to them, and what grade is next to each course on the transcript.