I am admittedly thinking of an interface from how they work in Java, so I may be making incorrect assumptions about how they work in Go, but wouldn't I have to do that anyway? In what sense can the compiler work out how I want to compare my types? If I hand it some vec3s, for instance, does it know I want them sorted by x value, or by length?

By defining an interface that assures the compiler each type has a compare(x) method, I can write a generic sort function that takes any two comparable objects and sorts them based on whatever the class of those objects decided was the ordering criteria.